I have a 1989 300SE, and the only time I had a no-heat symptom, the cause was the monovalve. This is located in the area between the firewall and the outer firewall, if that is what it is called. As I recall, it is to the right of the battery. Anyway, there is a rubber diaphram that can spring a leak, and render the part inoperable. The repair was simple, and I was able to do it myself. I suggest running a search on "monovalve," and reading up on it. It may not be the problem, but when I had the symptom, I did some research and found that it is quite common for this part to fail after years of service. As I recall, the repair kit was not too expensive. Good luck.

Joe B.