May sound silly but is there coolant and a thermostat installed also maybe there is a heater core shut off valve
Have you tried blocking the apu radiator with cardboard to see if motor gets hot
If heated by coolant, it is the heater control valve or as mentioned, the thermostat may be bad. If heated by electric, could be wiring, fuse, or a breaker. Can also be a panel issue, but the others that are listed would be more common.
