My 1992 GMC 1500 blower motor quit working all together.

On any other vehicle it sounds like the blower fan switch.
The fan switch provides power or ground to the resistor and there would be a high speed blower relay to bypass the resistor.
But in your case the fan switch is part of the controller.
I don't have a diagram for a 92 but the 94 model shows relays for low medium and high speeds. Each relay gets power from a fuseable link. The high relay gets power thru the low relay if the low relay is not engaged.
If you have the model with 3 relays you may need to check them.

check the resister next to the blower motor they burn out

1-4 speed are the blower motor resister . It controls all 1-4 speeds, also it's under the passanger side dash around fan, the 5th speed runs off a fuse the is located in the fuse box. It's one of those square fuses , not the relays but fuses. The resister will run $15 and fuse $5 . Good luck. I have replace these before

Run 12V directly to the motor and see if it will run, if it dosent than the motor is bad, if it does than the resister is bad, the resister makes the blower run and also regulates the speeds for the blower, but chanses are if it was making a high pitch squeel before it quit the motor went out.

Assuming that the dealer is correct and isolated the problem down to the blow motor...
it IS possible for the motor to work intermittently due to heat buildup. Malfunctions due to heat will call the motor to run fine for a bit, and then quit when the motor reaches a high enough temperature to cause a malfunction. It would then quit working until it cooled back down, and repeat the process.

Whether is it just dead, or it is intermittently dead, for all intensive purposes, the motor is no longer usable and needs to be replaced with a new unit.

Check the blower resistor pack - usually located in the heater air ducting in the blower fan airflow. Check for loose connections and open circuit resistors on the unit.
If speed 4 works always, its unlikely to be the blower motor ground connection.

probably a bad resisiter in the plenum

Sounds like your blower motor has quit if it is inop on all levels. If high speed works and the other speeds do not, then its probally the rheastat or resistor pack

The problem is probably the blower motor resistor. I'm not sure where yours is located exactly. They can be on the firewall under the hood or some are near the blower motor under the dash.

replace fan speed reostat,generally high spd. does not go thru the reostat-full 12v service..kev-mn.