First off, the "normal" failure mode is for the heat to be on full blast. The system is vacuum controlled, so any leak in the vacuum circuit can cause this problem. Begin by checking the various rubber connectors under the hood. There aren't that many in a gas car, so it's easy enough just to replace them all as a first step. If the defroster pod isn't holding vacuum, that's enough to screw up everything else. You will either have to replace the pod or put a stopper in the line to test.

There are no solenoids involved. There is multiport vacuum switchover valve located behind the dash, on the right side. It can be accessed by removing the glove box liner. Sometimes these fail, and produce strange problems.

The heater valve has nothing to do with regulating temperature. There is an air blend flap at the top of the heater box which mixes cold and hot air to produce the desired temperature. The bad news here is that the only way to replace a pod is to remove all or part of the dashboard. For the defroster pod, you may be able to get away with just pulling the instrument cluster. For the heater flap and recirc pods, you should be able to pull just the radio console. But for the air blend flap, the entire dash has to be removed. Here is some reference material:

Troubleshooting the Mercedes-Benz 190 Tempmatic ACC Climate Control System

190 Restoration (click on any photo to enlarge)