Vacuum unit, right side, under corner of the seat base. Right next to the battery. If the battery has been out for any reason, then check that the vacuum unit has been plugged back in properly. In fact, check anyway. Mine wasn't, and was operating intermittently.
Not sure which fuse it is, but then, it may be a little different compared to a UK car.

If the vacuum unit is running, then start searching for leaks.

There should be actuation from both front doors. Inside and out. Although I can't double check that bit right now, as my car is a few thousand miles away...

85 WR quattro
90 V8 Silver, 91 V8 Black, 93 V8 Lago
93 100e quattro V6. Titan, i think