Abe,
Does the fan come on when the pressure/temp switch on the reciever/dryer is jumped? If so, you know the fan, and relay are OK. To check the switch on the thermostat housing, remove the wire (I assume a single wire system) and ground it while the car is running - the fan should come on. If not, the problem is liely the t-stat switch.

Without a wiring diagram,I can't say how to add a manual switch to the fan. Assuming the relay is OK, you just need to find which pins are used to run the fan and run a switch to close them.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer