I took one of the MACK radios apart ,{I could get it going if someone out there wanted one}, the chassie and output are insulated from the case, transistors will burn out if polarity is wrong. The old vibrator sets the one's with the sychronce vibrators, used in 30's and 40's Buicks and others ,will not play on the wrong polarity, but will not burn out , I've had 1930's auto sets that have reaversable polarity switch, wires and vibrators , the reason these old truck radios will work with either positive or negative ground,when equiped with original vibrator, is that the only thing the 6 volt line runs is the tube filament and the power supply witch converts the DC voltage to AC voltage , then it is jumped to 250+ volt and rectified using the correct polarity for your B voltage
