I'd use a DPDT switch instead of a diode. It has two separate circuits controlled by one toggle. It will have six lugs on the back. Run the power for your direct-wired fan to one of the middle lugs. Run the compressor power feed wire to the other middle lug. Run the outlet wire to the electric fan to one of the end lugs on the same side as the fan power wire. Run the outlet wire to your compressor to the end lug on the opposite end of the switch from the fan outlet wire and on the same side as the compressor wire. You will have two unused end lugs, one at each end and on opposite sides.
Wiring it this way will isolate the compressor to prevent feedback. When the switch is toggled one way, the A/C unit will run as normal. That's your default position. When it is toggled the other way, the power to the A/C unit is disconnected and the fan will operate on its own.
Matt
