I've been playing around with some Arduinos I got over the christmas break and threw together a backup assistant. The prototype in this video uses just one sensor, but I plan to put a few of them in parallel to widen the vision angle.
Here's how I was thinking I'd power the sensor:
The receiver would be powered with a simple cigarette lighter, regulated to nine volts, and the vibration motor(s) would be installed in my car seat. I'm speaking in the second conditional here, because I may end up ripping these apart and using the arduinos for something else. They're so fun!