Full heat all the time and in all settings means that the monovalve is in its default full open position. The cause is either a bad monovalve or a failure of the circuit that controls it. As Sixto says, check the fuse first. Don't just look at it, remove it, hold it in your hand, make sure it isn't cracked across in the middle and make sure the ends are not corroded.

If the fuse is good, check for voltage across the two terminals of the monovalve with the CCU set for full cooling. If you measure 12 Volts, the CCU is probably OK and the monovalve is probably bad. If you measure 0 volts, the problem is probably in the CCU.

You can also test the monovalve by removing the plug and (for a couple of seconds only) put 12 volts across the terminals (either way). You should hear a click as the solenoid moves and you may see a little sparking as you touch and remove the wires. If the monovalve doesn't react, remove it and take it apart. Failure modes are (a) torn diaphragm, (b) jammed diaphragm, (c) rust in the solenoid coil due to an internal coolant leak, (d) open coil wire and probably others that I can't think of now.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970