1991 GMC C2500 Auxiliary / condenser fan inoperative

1991 GMC suburban R2500 7.4 TBFI, 2WD, auto trans 4L80E, with rear A/C. A local A/C shop had done the 134a conversion, and replaced the compressor, accumulator, front evaporator etc. Two weeks later it quit cooling. The shop said an O ring on the pressure switch blew due to overpressure because the condenser was not being cooled properly. The A/C did not work well in traffic or at slow speeds after the conversion. The electric auxiliary / condenser cooling fan was not working. The A/C shop wanted to replace it with an aftermarket unit ($200) since they said the fan motors were no longer available, which is true. The fan motor rotated freely so I thought it may just be a fuse, relay or switch of some sort. I declined the aftermarket unit. I had the truck in the dealer for other work so I asked them to diagnose the fan problem. They jumpered the fan motor at the motor and it worked ok. They said they thought it was a pressure switch on the compressor that no longer existed due to the replaced compressor. They suggested I wire my own relay and manual switch. HOWEVER, on the way home from the dealership the fan started working, but quit after a few minutes and has not worked since, under any conditions. I have not been able to get it to kick on! What, how, and where is this fan activated? I can't find a swith or relay in the system and neither could the dealer. It worked ONCE the way it is now, and surely we can simply fix this without an aftermarket unit.
Thanks, Mike