[Jared Russell]

Use a spare 3-pin white/red/black PWM cable and strip the male end (or use any small gauge wire for that matter).

Wire one terminal to ground (black on the PWM cable).

Wire the other to signal (white on the PWM cable).

Hook these wires up to a digital input using a 3-pin connector (the female end of the PWM cable).

The default code will do the work for you, assuming the switch goes in digital input 18 and the compressor is on relay 1.