Hey, Rustee ...I designed a similar setup for the Aux fan, but only used one relay with the fan's power taken from the N/O terminal (87). There are three triggers which can independently activate this relay: radiator temp switch; dashboard toggle, and AC compressor clutch wire. They all join at the 86 terminal of the relay, BUT each has a diode in circuit, anode towards the relay, to prevent current backfeed from any of the others. The relay is grounded (Terminal 85) by a connection to the Yellow output wire of the OD relay to the OD solenoid - again with a diode in circuit to prevent unwanted current backfeed.

Yes; the "OD grounding" is a logical feature - as you noted, there is unlikely to be a need for the aux fan at OD speeds.

I can't figure out how to do something similar on automatics - the OD activates hydraulically. You can manually enable/disable its functioning by switching off its solenoid via its relay, but that switch has nothing to do with car speed.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)