I took another look at the diagram and
the S25/5 IS only used on the early cars (86-87).
The later cars have the sensor B11 which gives actually temp reading to the control unit so that it can use programing for all the various functions it controls: high speed Aux fan and A/C clutch on siganal.
The fact that you have found the 85 terminal of the K9 relay to be grounded is key here. I would test the temp circuit at the pushbutton controller, but then again I have the testor. It probably would be easier under the hood. The B11/7 sensor is right by the engine lift bracket by the oil dipstick. If you have problems with this let me know. I am looking in the printed ETM and being that you have the last year of that chassis it is possible I'm missing a supplement. I have numerous other ways of viewing this if this isn't the way it is.
The problem could be the pushbutton controller but I would definitely check its imputs first.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician