So I have a 90 miata. The AC is removed completely. I like to do fun small mods. Ie interior lights, electric trunk opener, in dash accelerometer, Bluetooth..just to name a few comfort mods I have done......so with the AC out I'm left with a useless button on the HVAC control...I not only don't like useless things in my car but I think it riotous be fun to turn this little button into something fun..maybe air horn, fog lights, power up a go pro camra, flip down Licance plate who knows...but the first step is finding where this switch sends power? Where in the harness?

 

Externally, the A/C button provides a closure to ground on the Green/Red wire, but only when the fan switch is also set to a position other than "off". This is the wire which goes to the ECU (through the thermoswitch in the evaporator box) to command it to engage the A/C Relay.