So I have a wire on pin 86 of the high seed fan relays, that is connected to a latching relay, which is connected to the ground. The latching relay is controlled by a momentary OEM switch installed in the dash, to toggle the on/off of connecting pin 86 from the fan relays to ground, which turns the fans on/off. You can see the wiring diagram in this thread:
Defroster Switch Wiring
Back to the original topic, you can make most(all?) OEM switches work like on/off switches by adding a latching relay, but you should be aware that the not all of the OEM switches have the same internal wiring if you want things like night time illumination to work, or to use built-in LED's in the switch.