I have a 1977 Gran Fury brougham, and my AC/heater blower is acting up (or not acting at all, more to the point). It's peculiar because it's not a matter of complete failure: My AC (less than one year old) and heater engage, but the blower won't blow.
Did the blower quit entirely? The switch seemed to become quickly stubborn after a repair visit (unrelated matter -- turn signal problem) and worked a few times after being pushed very, very firmly. It hasn't worked since those few times. The blower showed no signs of faltering or weakening. My AC rocked right up until it stopped.
I'm trying to avoid taking it back to a repair shop -- it seems like something goes wrong every time I take the car to a mechanic; it's in great shape, if past its prime -- so I'm just trying to work through eliminating possibilities, so I can move forward with the repair.
When the fan stops blowing, does the blower circuit still have voltage? Check the fuse for the fan.
Check for voltage with a test light or a volt meter. Make sure all connectors are clean and tight.
If the fan is starting to seize; it will act erratically. Check to see how many amps the fan is drawing or remove the fan and see if it turns freely when moved by hand. If you can spin the fan and it stops immediately then replace the fan. It should spin freely.
The car is turned off when you turn the fan by hand.
Check to see if the ground to the fan is making a good connection. Remove the ground wire and clean it up as necessary.
Check the fan resistor for voltage in all speeds. The lower the fan speed the lower the voltage. You can also check the resistance for each fan speed at the resistor terminals. If you have any open circuits at any terminal then the resistor would likely be at fault. You might even be able to see if any resistor coils are burnt or open.
Check the fan switch also for voltage and for any loose connections at the plug. Repair as necessary.
Try these suggestions and let us know how you make out.