A/C heater fan stopped working

There are two versions of control system. If you have the manual system, with no test results to analyze, the best suspect is the thermal fuse for the fan motor resistor burned open as a result of the fan motor having tight bearings. The clue is the fan will still run on the highest speed. If it does not, it is also possible, but much less common, for the motor to be shorted. That would cause a 30-amp inline fuse to blow. To find this, start by measuring the voltages on the two motor wires when the ignition switch is on.