Left: Original Scosche TA-03 (came w/BlueBox) <------> Middle: Scosche TA-03HB (harness only/no BlueBox) <------> Right: Best Kits BHA8113 identical to orig TA-03 (search eBay)
Note: orig harness (left) doesn't have a shell covering the pins so it's easy to mod, newer harness (middle) has the extended shell covering the pins making it difficult to mod, and BHA8113 harness (right) is identical to the original TA-03 so GET THIS ONE!
20-Pin Toyota Wiring Harness w/RCA for 2K & Up Premium 8-spkr w/FactoryAmp Mod for:
The Scosche TA-03A & TA-03B Harness
and BestKits.com BHA8113 Harness
(update 1/05: This mod fits 2000 & up GT's & GTS's w/optional Premium 3-in-1 AM/FM ETR/Cassette/CD w/8 spkrs & separate amp)
= click to jump to post about which GT's this will fit & about TA-02 harness vrs. TA-03 =
(update 7/07: Latest notes on the new TA-03B Harness, BestKits.com BHA8113 harness, Metra TYTO01 harness)
1) Some time ago the original Scosche TA-03 was superceded by a newer TA-03B harness which uses a different connector that has an extended shell covering the pins, making it more difficult to mod.
2) When selecting resistor values from the Termination Resistor Table for the newer TA-03B, add +16% to the value from the table, or use the following calculator to find the exact value, substituting 2200 ohms for R1 (the TA-03A uses 1900 phms for R1): Voltage Divider Calculator
3) There are many wiring harnesses out there which claim to be compatible with the 00-02 / 03-05 Celica's but members have discovered are not compatible. We know the original TA-03 was the easiest to modify and absolutely works, as well as the newer TA-03B, even though it's a bit more difficult to mod. We also know the BestKits.com BHA8113 Harness also works, and may still have the original Scosche TA-03 style connector that's easier to work with.
4) Recently members have been asking me about a new "compatible" harness called the Metra TYTO01. At this time all I can say is that it looks like it might work with later JBL-equipped vehicles which use the Toyota databus to control Turn-On. Other than that, I don't know yet whether it needs the mod or not. Note: Metra does not list any Celica's in their TYTO01 compatibility table.
(update 8/07: )
1) Confirming: the BestKits.com BHA8113 Harness is identical to the discontinued original TA-03 harness, including long pins making it easy to modify compared with the newer Scosche, so I'd recommend you use the BHA8113 instead of the Scosche TA-03B.
2) If your HU only has two RCA outs (L&R) you can use an RCA Y-Cable to connect to the StockAmp's Rear RCA's, but you'll need to roughly double the value of the termination resistors to avoid double-terminating the signals. For example if the table says use 600 ohms, use 1000-1200 ohms instead.
You might think installing an aftermarket head-unit and hooking it up to the existing FactoryAmp would be a piece of cake, esp. using Scosche's TA-03 wiring harness and matching Toyota connector. Not so, and like just about everyone else here who's tried it I got loads of ECU noise, power-window buzz, dimmer buzz, and the worst: alternator whine. After installing and removing my HU about a dozen times trying things, I was about ready to throw that fancy HU out the window and put the stock unit back in, until...
I love my Celica like most everyone here does, and cuz I got so much help from reading the posts on this board, I'm posting this to give a little back. So I hope I do this right...
I'm an electronics engineer, but after re-checking everything it all looked like it was hooked up ok, aw crap. Like everyone else I thought it must be a grounding problem. Well, yes & no. And what's with the blue box Scosche gives you with the switch on the side? I wasn't about to give up because I know how good the stock GTS Premium Stereo can sound, so that FactoryAmp can't be all that bad... so I kept going.
In a nutshell... I found out our GTS Premium 4-channel FactoryAmp has high-impedance inputs with virtually no input termination. This means when you hookup most head-unit's PreAmp Outs (which have virtually no output termination) the RCA cables and the Celica's own HU-to-FactoryAmp wiring harness act like radio antennas picking up every little electrical noise generated by the car. Have you ever stuck your finger on the end of a guitar cable with the amp cranked up? Same thing.
On top of that, the TA-03 has a 1.9k resistor in series with every RCA cable. If you feel the heatshrink, you can feel the resistor inside each cable.
Solution... Terminate the inputs of the FactoryAmp to match the output impedance of the head-unit being installed. Doing this effectively dumps most of the electrical noise picked-up by the RCA's & wires to ground while allowing the higher-level audio signals to pass on to the FactoryAmp's inputs.
Click Here: If you don't see pics (goes to Tripod clone site) (updated link: 3/06)
Remove heatshrink from audio pins (notice hidden resistors in black shrink, also 4 new resistors in front
Back of TA-03 showing 4 audio pins - (Red Wire is Right Channel & Top Row is Front)
Unsolder the 4 RCA wires from the connector pins and twist termination resistors onto them before soldering
Add heatshrink tubing (yel) to prevent shorts & add structural support, then tin the ends and solder
them back to the pins
["tin the ends" means melt some solder into each twist first, then when you solder them back to the pins they've
already got plenty of solder so you don't need 3-hands to add more. Use one hand to hold the twist onto the pin
then touch the soldering iron just long enough to make the connection melt back to the pin.]
Add more heatshrink (blu) then twist the ends of the resistors together and tin with solder
Add larger heatshrink to Analog GND wire & solder it to the twisted end of the resistors
After applying heatgun to shrink tubing - (Top Row is FRONT Left & Right)
Completed Mod - tie-wrap all power leads together, then separately tie-wrap all audio & gnd's together
Scosche TA-03 FactoryAmp Noise Fix Mod - Basic Pictorial Diagram & Notes
The output impedance of my head unit is 60ohms at 5vrms, so the 1.9k (1900 ohm) series resistors don't hardly reduce any level and it's easier to leave em in. Basically what you're doing is adding a terminating resistor from each signal wire to ground, closest to the FactoryAmp's inputs. (Ideally you would do this right inside the amp, but this won't be necessary unless you plan on listening to your stereo while driving thru an MRI Scanner or past some military radar dish)
|__________________SCOSCHE SLC-4 "BLUEBOX" SPECS
|____________( comes w/TA-03 Kit - features & notes below )
| * 4-channel HV LINE LEVEL to 4-channel LINE-LEVEL converter
|__( don't need SLC-4 for HU's w/Preamp Outs - Only use TA-03 Conn w/Mod )
| * 4-channel SPEAKER LEVEL to 4-channel LINE LEVEL converter
|__( up to 80 watts max per channel )
|__( use SLC-4 together with TA-03 Conn w/Mod - adjust trimpots on SLC-4 to match levels )
| * Transformer circuit for noise cancellation.
| * Common ground adapter.
So what's with the blue box Scosche provides? Well in an ideal noise-free world you would set the switch to "Line" and plug your RCA's into it and wire the other end into your amp, but if you don't have a HU with high-level preamp outs you can connect your HU's speaker outputs into it (thru the screw terminals) and it will drop speaker-level down to line-level to feed the FactoryAmp. Will you still need to do this mod? Uhm, yeah probably... it's because inside the SLC-4 BlueBox are 4 level-dropping circuits and impedance matching/isolation transformers. What's great about transformers is that they can match the impedance of one circuit to another almost perfectly, plus they can isolate DC path between two devices and only pass the audio signal, but what's not so great is that they can also act like guitar pickups when designed to be connected to high input-impedance amps like ours.
Ever hookup a Scosche SLC-4 BlueBox and wave it around the inside of the dash with the ignition on? Acts like a Theremin doesn't it? That's cuz its transformers are picking up the noise from the data bus running all thru the Celica wiring harness. Ever play with a telephone pickup coil? Almost the same thing.
Notice the TA-03 has 3-ground wires coming out of it. Make sure you connect the ends of the 4 resistors (tied together) to the wire labeled Analog GND in the photos & diagram. The other two GND wires can be tied together and connected to your HU's GND wire. Believe it or not, once you terminate with resistors, your grounding problems will pretty much disappear.
I forgot to mention that when you terminate audio lines with the matching impedance of your HU, your frequency response will probably be restored more closely to what it's supposed to be too. In otherwords if it sounded too bright & tinny before, your bottom end (bass & lows) should come back to life again and you won't have to subtract your treble & max your bass eq settings anymore.
thanks for the help on the board & enjoy your FactoryAmp, I hope it helps...
ps ~ i want to give some credit to EvilED who posted a msg way back in 10-12-2002 about a mod for the factory amp. I found it while searching to make sure the TA-03 Noise wasn't a topic already covered. It just goes to show that doing thorough research pays off if you bother to do it (and no, I didn't find it until it was too late cuz I was lazy too... crap!)
Click Here: EvilED's factory amp mod
Click Here: If you don't see pics (goes to Tripod clone site)
Click Here: For a table of Termination Resistor Values and Final Test Results
Click Here: To jump to post about which GT's this will fit & about TA-02 harness vrs. TA-03
Click Here: Wanna hear a simulation of what the Scosche TA-03 Harness sounds like -without- the mod? [updated 4/2017: sample mp3 file moved to Tripod]