You might just have a bad cabin temp sensor or evaporator temp sensor.

These are the test specs for the cabin temp sensor:

15C = 15.7kO +/- .5kO
25C = 10kO +/- .5kO
35C = 6.5kO +/- .5kO

I can't find specs for the evaporator temp sensor. It might suffice to disconnect it if you know where it is. I don't.

It's easy to scrape off the silicone to get to the innards of the Klima relay. You don't even have to scrape it. Just get a blade between the floor and the case and pry.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD