If there is a light close by, you can probably fish wires from the light to the vent opening.
That is what I did on mine. Since mine was going over the bed, I wanted something quiet. I used the fan out of an old PC I had laying around.
Really low current draw, pulls out the hottest air, and I can't hear it running at all.
I put a switch in front of it, but it has been running non stop for several years.
2014 F 150 ecoboost
2008 Chrysler Aspen
09 Amerilite 21 (modified)
2013 Bendron 14' enclosed cargo
2011 4x8 open cargo