Thank you for all your responses.
Just to be clear: I tested the pos/neg on the harness leading to the fan. Voltage was 12V. Did NOT test the thin control signal wire. Read enough here to stay away from that. Got a brand new fan and plugged it into the harness. All 3 wires. Nothing. FSR, temp sensors, hoses just replaced (curiously this started shortly after.) Did that indy in Morris plains, NJ do it right? Some doubts.

What I don't know is what's coming out of the signal wire, and IF there's a signal to activate the fan. Upallnight, you make a good point about that. But already took it to the BMW dealer in Suffern who said I "need a new aux fan." RIGHT.

So, now to find an indy with diagnostic gear capable of testing the signal wire and more. This is a pretty pervasive problem. A lot of complaints, few solutions. Some replaced the Aux fan and that did it. Others never seemed. To find a solution.
I just want to use this resource to get some clues about what it might be to talk to a shop. Why reinvent the wheel?