Here is something that might help. It's a wiring diagram for VW's.
http://www.installdr.com/Harnesses/VW-Wiring.pdf
The deck has a little plastic harness that all the wires are plugged into. You should be able to find an adapter that plugs into that on one side and then into your stereo. It's cheap (~$20) and no tools needed for the job. Talk to the install guys at a Circuit City or Best Buy. Those guys do a lot of cars and are usually pretty knowedgeable. Since they don't work on commissions or get paid by the job they have no incentive to lie to you either.
I have a 2000 Jetta and I did it myself without needing any tools.
You can get installation kits here (you will probably need an anteana adapter):
http://www.installer.com/wires/index.html
Here is a step by step guide for a 95-98 Jetta (Passat may have the same stereo setup, the install kits are the same for all models I think):
http://www.installdr.com/InstallDocs/VW/PDF/890005.pdf
Another thing regarding my first post, the wiring diagram shows that the speakers are wired in parallel. Therefore, you do not need a separate amp to power the rear speakers. Just wire them in parallel.