Where is the thermostat located on a 1999 VW Passat?

It is mounted inside the water pump assembly which is essentially buried in the front of the engine.

When I recently replaced my timing belt I also changed my entire water pump including the thermostat, just in case.

Be aware that this is an entire day job for a do-it-yourselfer. I would suggest also changing the timing belt as you will be pretty much ripping apart the entire front end of your car to get to the waterpump.

Go to http://www.clubb5.com and check the forums. There is a forum called "Garage" that has a complete write-up of this procedure.

This site has a photo lineup, picture by picture guide to a few maintenance items including waterpump/cambelt etc - http://www.taligentx.com/passat/ and its easy to follow.Just like a colour Haynes manual.