If you can find it, clamp the lines that feed the heater core. That may reduce temps until you can find the problem.

If you can access the fan easily, unplug the wire. It's possible there's a pinched or shorted wire in there.

You may try a new thread with different subject line in Diesel Discussion and cross link to this thread. DD has a lot of A/C threads and your problem may be CCU related. Maybe something like "Need 1990 300E CCU help/wiring info"