Why does my 92 Mazda 626LX car heater and AC work on lower settings but cut out completely on full power?

Your '92 Mazda 626LX car heater and AC might work on lower settings but cut out completely on full power because of a faulty rheostat. The fan speeds will not work correctly when this part is bad.

Generally the high speed function bypass's the blower speed resistor. The high speed function uses a relay instead. I would start with checking the high speed blower relay.