Location on the 1.8 engine (including 2000 I believe): http://www.audiworld.com/tech/eng76.shtml (this writeup is for replacing the coolant temp flange, but it does a good job of showing the location on the 1.8.)

This gives info about the coolant temperature replacement itself (this writeup is actually done on a 2.8 however): http://www.audiworld.com/tech/eng76.shtml