On your 1988 chassis , there was a mid-year change on what sensor type was used. The thermistor type SB mentions replaced the earlier 3 prong temp switch, [ 105/115 sw]..which is just a temp sw [ not a thermistor type]
You can check your engine number to see which system you have , but the easy way is to just see if your car has a 2wire/BLUE sensor or a 3 wire/green sensor..
Here is the change engine #
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42AV18CUHG&year=1988&make=MB&model=560-SEL-001&category=R&part=A%2FC+ACC+Temp+Sensor
My bet would be that you have the earlier temp sensor and it is shorted, thereby giving you full fan and no ac.. and by you removing the connector , the fan stops b/c you have interupted the fan circuit..whereas , if you had the later thermistor circuit, removal of the connector would nor stop the fan b/c the CC panel has a full fan default circuit if the sensor is bad and connector removal would energize the fan.
Check and see which you have...........
If one has the blue thermistor type, you can unplug the connector and bridge the connector with a 1K ohm resistor ..that will give the CC Panel the required input to stop the fan activation circuit..which would indicate a bad/open sensor/thermistor....