Radiator fans run constantly, 92 100S (similar to A6). Suggestions?

At a guess probably the fan temp sensor in the radiator. It's usually a fairly sizeable item ((for a sender) though not too expensive), trace the wires back from the fans to the radiator, you'll find it. Don't undo it though till you have a new sender as you need to get the new one in pretty sharpish before you drop all the coolant.

You may also want to check the three fan relays to see if they are sticking. Two are located in the relay box on driver's side in front of the windshield under the hood. The third is under the dash by the driver's left knee. It is a big double-wid relay for the fastest fan speed. If the fan is very loud this may be the problem because the fan thermoswitch on the bottom of the radiator controls the slow and medium fan speeds.

The fan is controlled by switching the ground connections for the three speeds so it could be a ground wire from the fan shorting to ground.