Holux GR-239 is a well designed bluetooth GPS receiver that makes batteries and wires obsolete. How? It just plugs into your car cigarette lighter. No need to worry about if you charged your batteries. Not to mention it also is dual function; serving as a wired GPS receiver in case you are not into the whole bluetooth thing. It’ll just transfer GPS data via a USB cable in that case.
The rest is pretty standard: A high sensitivity SiRF Star III GPS receiver that can track 20 satellites at once and the option to use an external GPS antenna.
It sells for £60 in the UK and it doesn’t seem to be available in North America just yet.