There's a relay involved in the engine cooling fan circuit.
The thermostatic switch at the bottom of the radiator operates this relay.
I'm not sure where this relay is located, but I'd try looking on the left inner front fender (iirc, there a few relays located here).
Going by the only wiring diagram I have for the older than your U.S. 124 Spider, the wiring colors on the relay are:-
Terminal 87 - medium gauge Blue wire (this goes to the engine cooling fan)
Terminal 86 - white/black wire (this wire comes from one side of the
thermostatic rad. switch and when this switch closes it acts as the ground to operate the fan relay)).
Terminal 85 - white wire (goes to the stop light switch and also to fuse I or No. 9 on the fusebox - this wire provides the live feed to operate the fan relay).
Terminal 30/51 - double medium Violet colored wire, one goes to the air horn compressor relay, the other comes from fuse A or No.1 on the fusebox - this wire provides the power feed to the relay.
So, briefly, you're looking for a relay that has blue, white/black, white and violet wires going to it. If this has failed, you can probably use a standard aftermarket relay, just be careful you get one with enough Amp. capacity, iirc those old engine cooling fan can draw up to 30Amps on start up.
Some of these old relays had a metal cover that could be removed to examine the insides. If you apply 12volts to terminal 85 and ground terminal 86, the contacts should close if the relay is working. If you can't open it, do the same and listen/feel the cover to check if the contacts close, alternatively, also apply power to terminal 30/51 at the same time as the above and you should get power out at terminal 87. Another way would be to connect it as normal, turn on the ignition, join together the 2 white/black wires that go to the rad. thermostatic switch and the relay contacts should close.
I'd suggest you try to sort out this fan power supply issue asap, I don't think powering it from the ignition coil is such a good idea due to the high power draw on start-up - it might affect ignition output when the fan is running, plus the ignition wire feeding the coil wasn't designed to carry this amount of current. I did note you had fitted an in-line fuse - always a wise precaution. I'm curious as to what amperage of fuse you fitted, if it hasn't blown, then the fan has to be drawing less than it's rated for and my vague memory of 30Amps on start up could be wrong, but iirc the fan draws less than 10 Amps when running.
It's unlikely there's any problem with either of the 2 fuses I mentioned above (A and I or, 1 and 9) as these fuses iirc serve more than one item, so if it's only your engine cooling fan that doesn't work, it's unlikely to be a fuse issue.
I'm not sure how you wired up the fan to get it to work, but if checking the above doesn't help, you might check that the black wire from the cooling fan goes to ground, iirc the black wire is grounded at the voltage regulator mounting, this black wire is also connected to one side of the rad. thermostatic switch (both wires going to the rad switch are white/black).
Be careful working near the engine cooling fan as it may suddenly turn on!
Hth,
Al.
