I couldn't find your model, but according to Page 47 of
this manual on BryantRV, the colors are:
Thermistor = White/Black and Red/White
Light = Red and Black/White
Wish I knew what turns the Light On/Off. In our N811, I don't remember seeing a Switch either, and I know the Logic Board turns it Off after a period of open-door time.
When we had an older Dometic, with a Switch, I installed a fan and I think the way I did it was go across the Light Switch.
Is there enough space in the little chase the four wires you describe run in, to be able to run two more skinny little wires?