Not only were cars equipped with 6-volt systems, many, especially British cars, had positive grounds.
Yep, the positive-chassis survived into the 12-volt era of the early/mid-1960s on many British models. Remember this was still very much the age of the dynamo too; alternators didn't become common on cars until a few years later.
I used to do car stereo installs, back when 8-tracks (and even 4-tracks) were king.
I knew a guy in a village where I lived once who changed his car about as often as other people change their socks! Every time he had problems with moving all his radio/tape/CB gear across, just on negative-chassis versions.
Then came the day that I opened the door to find him standing there with a CB set in his hand, the "aroma" of burned-out circuit-board tracks being obvious even without removing the covers. He'd just acquired an old early-1960s Ford Anglia and hadn't realized that it was positive ground. (Nor did he realize that a CB set doesn't need a 35A fuse inline!)
There were converters that could convert 6 volts to 12 volts, and even positive ground to negative ground. Some brave people got around this by isolating the stereo chassis from the car body metal.
I think I did that with one transceiver. I ended up spending a lot of time playing with capacitors to get the live chassis at "RF ground."
