If you cut the car's radio connectors off the wiring harness, you'll void the warranty of the car (or worse). Trying to splice or "tap" the wires will get confusing even if you don't connect the blue wire with the white stripe to the white wire with the blue stripe.
Do yourself a huge favor and purchase a wiring harness adapter for your 199 Passat instead. Crutchfield sells one for your car
for $10. Likewise, most electronics stores that deal witrh mobile installs have them too - check out Best Buy, Circuit City, Fry's and even your local independent radio shops.
You wire the new radio's harness to the harness adapter for the car in the comfort of your home (the light is better and you don't have to contort yourself to do the work). When complete, bring it out to the car and plug one end into the car, and the other into the radio. Insert the antenna plug and you're done wiring!
If you want a wiring diagram, those usually aren't available online for free. You need to spend a few dollars for a Chilton's Auto Repair Manual or similar at an autoparts store. You may find registering at a site for Passat Enthusiasts will be beneficial.. all the folks there have Passats and discuss everything from tire sizes and rims to transmission rebuilding. One such site is the "Forum
" section of Passat World.
I hope this helps & good luck!