I've been able to charge it with my cheap, generic wall-charger, and my Galaxy S2 charger, haven't tried it with my car charger yet. My guess is that your car charger delivers too little current, too high or too low a voltage, or lacks the resistor across the data pins that tells the tablet it is a charger and not a real USB host device.
I have the same issue. Charges off a wall - USB charger or a computer USB port, but even with the same cable, won't charge in the car.
yes I have the same problem. I have 2 cigarette lighter adapters with 2 amp outputs on them and neither will charge my nexus 7. They both will charge my ipad 3.