Since you have VCDS/VAGCOM; what are the trouble codes, it should have specific codes and you would be amazed, what specific faults it can tell you (if there are codes). Post up those codes; they can point you in the right direction. I would start by getting a test light/circuit tester and looking for power, once you find power... look for problems beyond that point in the wiring. Here are some factory manuals online; that can get you started on wiring diagrams and locations, testing procedures for electrical components:
Volkswagen Workshop Manuals
Volkswagen Workshop Manuals