Audi Repair: Cooling Fan?, thermal switch, audi a4

Question
My car's fan (don't know which one) comes on every 4 minutes, runs for 30 seconds and clicks off.  This is when it is just sitting in the garage, no keys in it.  It has drained the battery.  What's going on? It is an Audi A4 1998.

Answer
The main cooling fan runs directly off the battery, and is triggered by a thermal switch, usually at the bottom of the radiator.  I believe there is also a relay involved.  In order of suspects, I would suspect the thermal switch first, wiring second and relay third...  Let me know what you find,  Jan