AC light comes on when I turn on my heater

Check out the other post I quoted. From the first PDF (on the left) on the first page it looks like by 2000 they put an switch that would close when defrost was selected. I'm thinking of that one was not allowed to close when you selected defrost it would allow normal push button AC and not switch on the AC when using defrost. Whatchya think?

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Also yeah, I believe that would be correct, neither switch should stop the other from functioning, they're parallel.

 

So it looks like you can do this without hurting the AC button's function for when you'd usually want AC on. I'm assuming this means you could turn it on if you needed it when the vent setting is on defrost and when it isn't. Here's my take on a diagram:


So if you take out the control plate with the vent control switch you should see a wire and I'm guessing you could follow it to a switch that hopefully will use a connector you could unclip and see if my theory works. If it doesn't, you can just reclip the connector to the switch. You could also try cutting the wire and taping off the ends, but just know how to solder in case it doesn't have the desired result.

 

The blower motor not working on the low setting is probably a bad blower motor resistor. It's a separate part from the blower motor.