I had same problem - one day there it was - all wet and I thought how do I clear it.
So hear is the best solution.
1. Get hold of a heat gun (paint remover type)
2. Remove the small side light (engine side) - it twists to unlock).
2. Get a copper elbow (22mm) and then put in a straight copper tube (22mm) about 6 inches long.
3. Put a plastic funnel in the end of the 22 mm tube (I used the plastic rose from an old watering can)
4. Make sure all joints for tube/elbow and rose are good (sealed).
4. Fit the 22mm elbow into the socket for the bulb (you just removed).
5. Get a vacuum cleaner and put the suction hose into the plastic rose (or funnel).
6. Apply some heat to the lens with heat gun (not too long in one spot - when the condense starts to vapourise, use the vacuum cleaner to suck it out. Takes a while - so be patient.
I did it and its been fine since and several car washes too.