If you have a ceiling light nearby, you can tap into that circuit. If the light is part of a series that operates with a wall switch, you may have to turn that set of lights on to operate the fan. If the lights can be turned on/off at the light then you can just leave the wall switch on.
Once you remove the garnish ring (trim) on the inside of the vent, you might be able to “fish” the wires from the nearest light. There is a space between the ceiling and the roof, that’s where lots of factory wiring will be. When you find the wiring, your looking for 2 conductor stranded wire. One side will be white (ground) and the other side will be white with a colored stripe (positive). Test light will verify if it’s constant or switched. I use male plug with a long “tail”attached to ground prong. Since ALL grounds are common, I just plug in to the nearest receptacle, attach the ground of the test light to the tail and then use a probe to poke the positive wire. The light will light up/buzz when 12V is present.
MAKE SURE THE WIRE IS ATTACHED TO THE GROUND OF THE PLUG, NOT THE HOT OR NEUTRAL SIDE.
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.