To keep in the car? Like a car phone?
Or just a small vacuum to use inside the car?
For small messes I use my Dirt Devil Black Scorpion model. Like when my kids spill their bag of cheerios on the floorboard, it's quick and easy.
When I am vacuuming the whole inside of the car out, I use my Shop Vac. Just plug it in and use the fitted attachment. Very powerful.