I am hesitant to give advice without being there to confirm myself. I would measure from chassis to each of the wires to confirm which ones are common to ground... and depending on the digital meter it might go instantly to zero or give a value then go to zero. Giving advice without being there is.... what... touchy... talking long distance here, I would prefer an analogue meter with it's instant response but given a zero reading on your digital... allowing for meter lag time... to the chassis, I would say yes, that is ground and they are common. About the switch... yes, the three position switch would work fine but without the center off position it would have to be a break before make type, which is usually stated on the package. If I was doing it I'd use a relay with a trigger wire to select... relay with enough contacts to provide the switching that break before make... but the proper switch will work just fine. It is important though to insure the speaker output grounds are actually common to the chassis... if so switching just the positive side is okay... if not they, all speaker wires, have to remain separate for both channels through the switch to the speaker for the 8 track. I'm quite certain the Panasonic will have common ground wiring for the speakers but check to be absolutely sure... common ground for the speakers means common ground for everything including power ground....as it is with the factory radio. Wish I was there to help... love those cars... had a 66 factory Hi Po 289 for years... drove it all around the country... had highway gears, great road car.
