It may be possible to use the switch only on Hi. Using the Lo would supply less than 12V to motor and may or may not be OK. The 6V heater switch should be built for higher 6V amps, therefore OK for 12V if using a relay. Fuse the circuit very near the amps of the fan. As you know, the relay has big contacts to handle the startup load of the fan motor and the 6V switch would just turn on the relay.
My comment was about not using a switch and rely on user to self control the heat. Your lag time would be a problem, you would watch an old temp gauge to determine when to engage. Using a cab switch is not the normal way to go. It your case, in would be OK, you don't already have an overheat to solve. You are just installing a safety.
It may save a lot of effort to wait and see if it overheats.
I now understand what you want to do and with what parts. I'm slow. Thanks Robert.
