I think that electrically, the 99 Passat is the same as a MkIV Golf/Jetta/Beetle, although you probably want to confirm that..

Anyway, the Golf has a couple of things to watch out for;
Constant 12V and Ignition 12V are swapped in the factory harness's ISO connector. The easiest way to deal with this is to swap them in the empeg's harness too - the factory harness would cost about $3k to replace if you buggered it up, and if you ever sell the car then it's harness would still be stock.
The stock harness also has the OBDII K-wire fed to the stock HU. You want to double check that the empeg's harness doesn't conect anything to it. (It shouldn't IIRC),

Try searching on Golf / Jetta / Beetle for more information, there's been a few VW installs..