What could cause the heater blower to stop working even after it and the heater control was replaced on a 1992 Ford Thunderbird?

You still have a blower resister assembly, wiring, and a blower motor itself, in that system. Any one of those could have a problem. You need to use a test light at the blower motor connector and see if you are getting both power and ground. If you are missing a ground, most likely you have a bad blower resister assembly. If you are mssing power, check fuses and wiring. If you have both power and ground, and the motor isn't working, you need a blower motor. I have also found that sometimes tapping with a screwdriver handle on the motor will make it come on, and if that is the case, then again, you have a bad blower motor.