Ok, so I just wired up a pair of subs and an amp in my trunk. It was the first time I've ever done anything like this, but since I've done all the other work on my car I thought I'd take a shot at it.
I'll start with the problem: I'm getting feedback from the sub when I turn the radio off, and they also hit whenever I close the door, knock on the car, or from any strong vibration while driving.
Everything went together very smoothly. I'm still using the factory head unit, so I have a small converter wired up to convert over to RCA output. Power cord is wired directly to battery through firewall and tucked away etc. I grounded the amp at first to a screw behind the driver side plastic in the trunk. Everything started right up and had all the amp levels turned to lowest settings. But I noticed when I shut the drivers door, the subs would hit even though there wasn't any audio. So I turn on the radio and things sounded great, but when I turned the head unit off, I got hit with massive feedback until either I turned the car off or turned the head unit back on. I figured it might be interference coming through ground wire, so I relocated it to one of the bolts attached to the trunk floor that have luggage hooks attached to them. This location seemed to do much better, but the problem still mildly persisted.
The factory-to-RCA converter box had level controls on it, so I turned them down to lowest setting and that seemed to solve the problem. Now i'm noticeing the same problem again, but on a much smaller scale. There is a faint hum coming from the subs when the car is ON but the head unit is OFF. And the same goes for shutting the doors: small sub hit.
Now obviously since I'm no audio wiring expert I probably should have let a professional do the job, but I like doing things myself. I think I have things at least 75% correct if not more.
Anyone have any suggestions? If pictures would help, I can snap off a few in a day's time. I'll try anything.