I have an 02 SVT also...
I have an 02 SVT also and i've missed my sub since my Audiophile broke and i got an aftermarket stereo.
I always thought the 3rd harness was for the steering wheel controller so i left it hanging.
I'm anxious to know how to do this also so i'm going to take a picture of the 3rd harness for you tomorrow and post it ASAP.
Another problem i'm having is i just bought a Pioneer DEH-P5000UB head unit and i just installed it today, the plastic dash adapter i was using with the last stereo fits except i can't use it because the faceplate opens downward so you can put a CD in and the adapter is keeping it from opening. Is there an adapter that fixes this problem? I asked an installer at Best Buy and he said this is a problem with Foci and VW's but he never heard of a solution.