Radiator fan!

First, have you actually unplugged the j horizontal white connector at the bottom of the fuse-relay panel? The connector frazzling that usually causes this problem is not visible when the j connector is plugged in. Or do you have the SRI fuse-relay panel?

The relays that run each radiator fan motor is triggered by the same single "sensor" (so that sensor, which is a thermoswitch, is not your problem). The thermoswitch is located in the bottom of the LH radiator -- if you short the two wires together (simulating a "hot" condition and closed thermoswitch) both radiator fans should run when the key is "on".

(If you don't have the SRI fuse panel -- which I believe has the radiator fan relays moved to the backside) You can try swapping the two radiator fan relays to see if the problem moves to the other side. You can also remove each radiator relay and, with key "on", touch a jumper wire from the 30 metal female terminal in the relay socket to the 87 metal female terminal in the relay socket like this:

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when the jumper is in place (and the key is "on") = the corresponding radiator fan should run.

 

That's good -- Guido's fuse-relay panel provides/keeps easy access to all the relays.

This surprises me as I thought one other good reliability change on Guido's board is changing the female terminals in the white harness connectors to the much more reliable Spade/Faston type (although that still requires good wire crimping to the terminals). Are you saying that this terminal frazzled with the updated female terminals installed? Or did you not change the female terminals and still have the stock 2-prong "tong" female terminals?

 

Guido's 512TR boards are plug and play. The Testarossa boards need the larger terminals.

 

Oops, my bad -- I didn't realize that I had the wrong F model. However, I can't recall ever seeing the female terminal type on a 512TR -- is it a Spade/Faston type design or something else? (My guess would be "something else" as the removal/insertion forces for an 8~10 pin connector with true Faston female terminals would be huge -- but it must be way better than TR if Guido didn't change it, and I can't recall any pictures of "frazzled" 512TR connections.)