[VetteVet]

You are on the right track and if you have no brown wire from the ignition switch then you must not have any accessories and the heater switch will not work. If you have the brown wire from the ignition switch to the fuse panel but you have no brown wire from the panel to the heater switch then the same thing. IOW no power to the heater switch. AS you have found out , if you jump power to the heater switch you can make it work. You can run a wire from the heater switch fuse 10 amps, to the heater switch and it should work.

The harness will plug into the heater switch and the terminal that's left is the one the jumper wire will go to. It should be common to the rest of the terminals as you switch between them.