I used "Metra 70-1784 Radio Wiring Harness for Audi 88-99/Volkswagen 80-Up " from amazon, I also got "Metra 70-1786 Radio Wiring Harness for Mercedes 94-04/Landrover 99-04 " and was trying different ways of connecting to the amp. I ended up just using the speaker wires from the front out of the Kenwood and did the best bang for the buck mod for the subs. While it isn't perfect, I would prefer separate controls for the subs, it is VERY nice sounding.
I also used "Metra 40-EU10 Antenna to Radio Adapter Cable for Select 2002-Up BMW/Volkswagen Vehicles " and "Metra 86-9001 Radio Removal Tool For Select 1998-2006 Volkswagen/Audi/ Mercedes "