You asked simple, so run wires connected to each lead of the low pressure switch on the receiver/dryer to a manual switch in your interior. Flip the switch, and the fans will be on at low speed

Sort of related, I ran wires connected to the temperature sensor on the block for high speed fans. The sensor kicks them on when engine temp over 105, but I wanted to run them in the summer for about two minutes before the engine is turned off to keep the temps at 90 or lower. I used a MB overhead light switch installed in the center console as the manual switch.

The post above is a cleverer idea though.