I did as 's130' suggested
and checked out a 124 Spider electrical wiring diagram.
There's also a
relay in the engine cooling fan circuit.
This, iirc, should be located on the driver side inner fender - I'm not sure which one it is but the cable colors going to it and the associated relay terminal no's are :- Blue- 87, white/black- 86, white- 85 and violet- 30/51.
Because of the inclusion of this relay, the way the fan is switched is a little different to what you might have been thinking.
In short, when the engine get too hot, the radiator switch provides a ground to the relay (white/black cable@terminal 86) which then closes and supplies power (via terminal 87, blue cable) to the fan (blue cable). In other words, the fan is switched on by the relay supplying it with a live feed via the blue cable.
The other cable (black) on the fan motor is always grounded.
The remaining 2 cables on the relay, violet,terminal 30/51 is a live feed from the fusebox; white, terminal 85 is a live feed to the operating coil in the relay.
The usual way of checking out the radiator fan switch is to join the 2 wires (both are white/black) together - if the fan then runs (iirc the ignition needs to be turned on?) the rad. fan switch is faulty. Alternatively, it can be removed and placed in boiling water to check for continuity across the 2 terminals, as described above by' bugsymike'.
The fact that you can get the fan to operate (I'm not sure what you mean by 'touch the 2 wires of the fan and set the ground?), suggests that the fan, fuse,
most of the wiring and probably the relay are o.k.
So, what to check? There are 2 wires going to the rad. switch, both are white/black.
One of the white/black wires goes back to the relay (terminal 86). If this wire or terminal is grounded and the fan works, then this wire is ok, also the relay is ok.
The other white/black wire should be grounded, this white/black wire appears (on the circuit wiring diagram that I have) to be connected to the black wire going to the fan motor, both of which which get their ground at the *alternator voltage control box* (large black box on l.h. inner fender) mounting. I would suggest you examine this wire and it's ground.... Also check for ground continuity on the black wire going to the fan - this should also be grounded at the voltage regulator mounting. The simplest solution might be to simply connect both of these wires to a new ground point.
(*I don't know which year your car is, if it's later than my wiring diagram, it's possible that the alternator has a built-in voltage regulator, in which case there won't be a large black voltage regulator box on the inner fender,* but I'm doing the best that I can with the info available to me).
The 2 temperature sensors on the cylinder head have afaik nothing to do with the operation of the engine cooling fan - one is for the coolant temp. gauge, the other is for the high coolant temperature (aka engine overheating) warning light.
Hth.