That little diode thingy is called G = diode (seat adjustment).
I have no idea what it does, but maybe it helps with motor reversal as you go backwards/forwards.

If your fans run all the time, do they run in low speed (thru the big resistor) or high speed (loud enough to wake the dead)?

High speed is triggered by the water temp at 105 degrees C. Or if an open circuit is detected (failsafe).

If it's high speed, the water temp sensor switch on the head is probably bad, or the wire has broken. I don't know where the sensor is on the V8, but it will probably be on the head where it can screw into a water jacket. It will have two wires. when it fails or a wire breaks, it turns the fans on high (failsafe design). Maybe that will give you a place to start.

If the fan is running on low speed, removing the D relay should kill it.
This one may be more difficult. It is designed to to turn the fans on low when the AC head pressure is between 15-20 bAR. It is triggered by the pressure switch on the AC accumulator. Maybe that pressure switch is bad? You could unplug it when the ac is not running and see of the fan stops. if so, its a bad switch.

Hope this helps. Pardon the grammer errors, relaxing with a glass of chianti after a hard day - HeHe.

DG