What was found
(CAN) A/C HAD HEATER OVERHEAT LIGHT INDICATION. OVERHEAT SWITCH RESET. HEATER GROUND RUN & OVERHEAT INDICATION AGAIN. OVERHEAT SWITCH REPLACED. HEATER GROUND RUN FOR 5 MINS, CHECKED OK. NEXT GROUND OP BY PILOT RESULTED IN SMOKE IN COCKPIT. HEATER FIRE BOTTLE DISCHARGED. INVESTIGATION REVEALED HEATER BRUSH & CAP HAD FAILED RESULTING IN COMBUSTION BLOWER MOTOR STOPPING OVERHEATING CAN. WIRING NEAR MOTOR MELTED RESULTING IN COCKPIT SMOKE. FAILED BRUSH/SPRING VERY WORN COMPARED TO OTHER BRUSH. HEATER'S LOW TIME & SUBSEQUEST FAILURE OF COMBUSTION BLOWER MOTOR MAY MAKE NECCESSARY A BRUSH INSPECTION BETWEEN HEATER OVERHAULS.