I believe that the control is on the center console by the shifter. It's usually on the door panel under the mirror itself, but I'm sure that you already looked there.
The best thing to do is check the glove box for an owner's manual. This should show you how to adjust them. If it's not there, then call the rental place and see if they can help you.
Of course you can always contact a VW dealer. Good luck.
〉 Answered on Oct 6th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines