If the hvac cabin fan is on and you hit the a/c button the a/c fan comes on, or if you have it in the defrost position, no need to re-wire anything as long as you're using the same wiring from the fans/relays.
I believe it wont come on with just the button.. It needs high pressure from the AC coolant. Which is no longer there.
Fan works regardless, it's the compressor clutch that won't kick in without the switch.
