My heater is stuck on ---Could someone please tell me where to start in solving the problem. I have an "89" 300 E[8D]

 

When you say your heater is stuck on. Is the blower on, the blower heat on, or just the heat. Or the AC is on If the system is vacuum operated, try reving the engine to 2000RPM a few times, that will evacuate the vacuum and then press the off button. Make sure you have one of the on button pressed first.

If is is electric, they there could be a bad relay, bad switch, bad contacts in the relays. Try disconnecting the battery. As most of these early system wer vacuum and electric. Disconnect the negative cable and wait a few minutes, press the off button or switch. Then reconnect the battery and see what happens?