As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE (HEATER FAIL) ANNUNCIATOR ILLUMINATED FOLLOWED BY AN OVERHEAT OR BURNING SMELL IN THE COCKPIT WHILE IN FLIGHT. RETURNED TO DEPARTURE FOR AN UNSCHEDULED LANDING. MAINTENANCE FOUND THE INSULATION ON THE WIRES ON THE HEATER MOTOR HAD OVERHEATED AND WERE HARD AND CRUMBLY. THERE WAS NO EVIDENCE OF THE WIRES SHORTING TO GROUND, HOWEVER SUSPECT FAILURE OF THE NOISE FILTER OR MOTOR ITSELF. POWER AND GROUND WIRES FOR THE MOTOR OVERHEATED. (K)