Or you could use on board telemetry , and Brian G's idea turning an old servo into a remote switch with your third channel.:lol:

The Nomadio Sensor I use does support telemetry, but I use all Spektrum receivers, so the telemetry won't work.

I don't mind having the fan run full-time, I might not even end up needing it all all. But it would be nice if I could get it working temp-controlled like the regular MMP and let the controller decide if/when the fan needs to run.