Several .. First inspect the motor, it is where the co-pilot can kick it, Make sure the wires are attached (I have a real issue there)
now... This is on my workhorse, yours might be different but down near the break pedal is the IP fuse panel.. One of those fuses is "HEVAC" or Heater... Mine was NOT connected to it.. However if you inspect the wires connected to the nearby terminal block (IP BLOCK) you may find a wire marked HEVAC.. I did, it was yellow as I recall but do not take my word for that, Clearly lettered.
It was connected to AUX-1 Terminal.. and that is a circuit breaker (The lower one on the left of the Fuse block) which was defective.