It is a 12v 120mm caravan vent fan with a PWM speed controller.
It is installed on a piece of clear plastic the same thickness as the window glass. To install I put the window down, bent the plastic slightly and put it into the window frame, then roll the window up underneath it.
The power plugs into the lighter socket on the dash. I have wired all the sockets on the car to the 2nd battery so they are always on.
Even at a low speed the fan prevents the inside of the car from becoming damp overnight with 2 people sleeping inside. And if it is damp, this can dry it out.
We have mesh on the rear windows so they can be opened a little to let the air flow through the car without bringing bugs and things inside.
I have not tried it is summer yet but it should also help reduce heat.
One thing I plan to do is replace the fan with a high-end PC fan. This should be quieter and allow for lower speeds when only a small amount of air movement is needed.