For cars with manual climate control, if the fan doesn't work at speeds 1,2, and 3 but works on 4, most likely it's the resistor. You could also test the blower motor with a 12 volt source. If you touch wires to the blower where the wire harness connects and to your battery and it spins, then the motor is good. The resistor and the blower are close together so if you think it's one or the other, I'd try testing the blower and if it works, I'd try another resistor while you have the glove box and knee bolster out. Make sure you hold on the the blower motor firmly if you are going to test it so it doesn't spin out of your hand and break the fan cage if it does turn on.

On 850's up to 1995, you can get fault codes for climate control from the B1 port on the OBD1 diagnostic helper under the hood. Hopefully, the little wire that has the probe which you insert into the different ports to get codes isn't broken. I have 2 850's and both cars have worn wires where the probe wire meets the OBD1 unit itself. Be careful to not tug the probe too hard (from the OBD1 cover which stores the probe or the different ports). The wire is short and you may tug the wire from the unit if the diagnostic probe pops out with force.

Also the resistor for cars with auto climate control are different from those with manual climate control. So if you're gonna get a resistor from the wreckers/scrapyard, make sure you pull it from a car with the same system as yours.

Hope it's not the control panel itself that is faulty. They were $1200 from the dealer and the part has been discontinued (at least in Canada).