Here's the North American tail light wiring diagram for 1996 and 1997...:
...the German Vento tail light wiring diagram...:
...and the German Vento rear fog light diagram:
The rear fog light is Nebelschlussleuchte, abbreviated NSL in the lower-right corner of the last diagram.
Looks like it's wired into the #12 terminal of the European light switch; no relay.
I hooked all this up on our 1997 Jetta 12 years ago.
The single rear fog light goes, of course, only in the left-side tail light housing. I had to drill out a hole in the reflector for the bulb, but I seem to remember that the bulb holder was all there ready for the bulb. I did have to run the +12v wire up to the switch, though, under the trim under the doors.