I think what you have is a pigtail meant to go through the door or window, and that would plug into something installed inside the car, maybe a socket on the headliner that is wired to 12V. Looks like the kind of plug that is used in lots of low voltage power supplies. Shouldn't be hard to find a socket that mates with it, and you could run a wire from there to under the dash, to a suitable (fused) power source.