I want to rig up a switch or something that will prevent the a/c compressor from coming on when the defroster is on. Where would be the best place to tap in? Sometimes I just want fresh air to blow instead of the a/c kicking on

 

I'd say splice your switch into the wiring AT the AC compressor. There should be 2 wires leading to it ( 1-voltage and 1-Ground ) cut 1 of them ( I'd cut negative ) & run the 2 wires for the switch into the cockpit and mount where-ever you like. When you turn on the Defroster just flip the switch to kill the voltage ( or ground ) directly feeding the AC compressor.

I should have put WAY more time than 2 minutes trying to figure the best way to do this,...... ( relay ? wire gauge, voltage spikes, ect... ) so after 2 minutes you just got the simplified 'hacker' method ! I would recommend getting the HVAC schematics that are specific to your car for review before cutting anything.

 

Why not just disconnect the plug by compressor?