I'd suggest getting a power inverter instead--after all, the car is going to be carrying it so it doesn't matter if it's heavier or bulkier than a charger. It will also work with other devices. I just leave mine permanently tucked under the passenger car seat until needed...
I've got several car chargers leftover from previous generations of powerbooks, each of which cost at least $70 and each of which doesn't fit other generations of powerbooks. Plugs are different, voltage requirements are different. If I'd gotten a power inverter right off, I'd have been ahead from the beginning.