I recently bought an 84 300 D, my first Mercedes. The heat rarely works. I've been reading this site looking for suggestions and it seemed as though the monovalve was my problem. I just installed thr repair kit and no change. After giving it some thought, it makes since that would not have been my problem. My problem seems more like it could be a short in the fan. It will come on at times, and when it does it blows hot, it just rarely comes on and then will shut off on its own, even on high defrost. Another strange quirk, the radio had giving me problems of not wanting to come on, however that has not happened lately. Could they be related? Any suggestions welcomed.

Both of those are grounded to the same spot up and under the dash mat. Best to check the ground continuity before going there. Should be able to check the ground wire against a good ground and see if it shows open. Check both the ground wire for the heater and for the radio.
Also check those fuses. MB as most cars, has fuses that are prone to getting dirty- corroded-whatever and failing to conduct properly.

have you checked the fan brushes?

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Paul
2004 E500 4matic; 72,000mi