Well the poster obviously didn't wire the setup correctly. The current for the fans should have never made it to the bulkhead connector or used the factory wiring.

When I updated my 51 to an electric fan I didn't use any of that wiring. I had already converted it to 12 negative ground and completely rewired the car about a year earlier.

Main feed for the fans came right off the 12v alternator output stud using it's own dedicated wire protected by fusible links (one line for high speed, another for low speed).

Those fed two high amp relays, the low speed relay uses switched 12v on one side of the coil and the ground side was controlled by a dedicated temperature switch, one set to come on at 185, off at 175. The switched 12v goes thru a three prong oil pressure switch (same as used for the choke circuit on an 87 Diplomat) that only applies the switched 12v to the relay if there is oil pressure present, in other words the engine has to be running.

Derale tells you to make sure the low speed is on before you turn on the high speed, so I used the output side of the low speed relay to send 12v to the high speed relay coil, the ground side is controlled by a dedicated temperature switch set to come on at 200, off at 185.

I have a bypass toggle on the dash to turn on the high speed fan if I choose to, but only if the low speed is already running. It will also be turned on by the AC compressor, when I get around to putting AC on it, lol.

I run the alternator from an 87 Diplomat, 78A. My setup draws about 26A at high speed.

Done properly there is no issue.