Have you checked the blower motor? When I bought my 90 420sel, they had had problems with the blower motor and when I got home the next day, it quit. In checking it, the connection where the blower motor ties in had come undone. When I connected it back, it blew and has blown ever since. There is a way to bypass the resistor(porcupine) and check to see if it is the problem. On my wife's 89 300se, the blower blew, but not very hard. A checked the voltage and it appeared to be within specs, but when he bypassed the "porcupine", it blew like crazy. I replaced it and now it blows extremely hard. I don't know if this will help any, but it is some food for thought.

Scott