I'd advise you to check into the various comercial mapping programs. Does roadmap do routing yet? I've got a Mobilecrossing Waypoint, and it's at the same time really easy to use and nearly impossible to use while driving. RoadMap on the empeg at least is a great hack, but it's not really usable.

As to the car battery, I suspect it'd power the WAP for a while without a problem, someone else can probably do the math for you.

Matthew