Why did my meter read 6 volts dc in the speaker output of a car radio?
Not odd at all. Car stereos don't necessarily have ground and positive for the speaker, they can drive both wires in a H-bridge configuration. Both speaker wires idle at about 6V, and there would be 0V over the speaker, and this way it is possible to drive up to +12V to -12V over the speaker. They usually say this in manuals too that speaker negative is not ground.