Not sure about your model but I suspect the solenoids (if they exist) are near the heater matrix under the dash. Someone else will kindly confirm it. I would just find the fuse in the fusebox and remove it to see what happens. If the heat is full on (when the dials are set to cold) then that just confirms they are de-energised and this is normal.
BTW, I assume you do not have air-conditioning.
1990 230CE 150k+ UK spec.