A transceiver would not be protected on the passenger seat. If you have a cell phone you could see that there still is some reception when you put it on a seat in the car. You could try the same with the ham gear but you may have to make sure that someone´s transmitting a signal. A metal box would to be HF proof if you want to be sure. A ground wire would help only if the car´s body is also grounded. I would at least put some foam in the box to protect the gear from shock.