Not only were cars equipped with 6-volt systems, many, especially British cars, had positive grounds. I used to do car stereo installs, back when 8-tracks (and even 4-tracks) were king.
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.
The reason they switched to negative ground systems is that the ignition spark is more efficient than with a positive ground system, because the spark jumps the other way.
