What was found
FLIGHT CREW SMELLED HOT PLASTIC IN AFT CABIN AREA WHILE HEATER WAS IN USE. THIS IS AN ELECTRICALLY ACTUATED SYS, OPERATED BY THE PILOT. IF PILOT SELECTED SETTINGS ARE TOO HIGH, THE HEATER CAN RUN VERY HOT TO THE AFT CABIN, SEPARATELY FROM THE COCKPIT, THIS WAS THE CASE. THE HEATER IS FUNCTIONING AS DESIGNED & PROBLEMS APPEAR TO BE ROOTED IN PILOT ORIENTATION, AS THIS IS A BACKUP ACFT CONFIGURED DIFFERENTLY FROM REGULAR PROGRAM ACFT. HEATER SYS HAS BEEN MEL'D & DISABLED. PARTS ARE ON ORDER.