Installing two radio systems (original in dash + new in glove compartment)

[Jeff Berk]

I’m trying to figure out how to hook up the original radio as well as a modern day radio in a 1974 260Z. The original speaker is gone and I have two door and two rear compartment speakers. If I chose to have the original (monaural ) radio power the two front speakers and still be able to run the two front and two rear with the modern radio, I’m assuming I need to do some special wiring since both radios cannot power a speaker at the same time and since the two front speakers cannot be hooked together for the original radio because that will be a problem for the modern radio that needs to keep them separate.

Please look this over and offer suggestions on if it will work or if there is a better way.

1)      Power from an ignition controlled source goes to a DPDT switch that toggles between powering the original radio and the modern radio (red line) and a power feed to a DPDT relay (orange line) or an open circuit.

2)      The original radio on the left has two speaker wires, the positive wire (white) and the negative wire (white with a black dash). The negative wire splits and goes to both rear speakers. The positive wire splits and goes to contacts A1 and A2 on the relay.

3)      The modern radio on the right has eight speaker wires with the rear speakers wires (green and blue pairs) going to the two rear speakers (RR and RL). The front speakers have two pairs of wires with the two negative wires doing to the two rear speakers (turquoise and white with black dash) and the two positive wires (turquoise and white) going to the B1 and B2 connectors on the relay, respectively.

4)      Assuming A1 and A2 are open in the de-energized state, the speaker signals will travel from the modern radio through the turquoise to B1, then to C1 and then to the front right (FR) speaker and through the white wire to B2, then to C2 and on to the front left (FL) speaker.

5)      Assuming the relay has been energized by the DPDT switch which is sending power now to the original radio, A1 is connected to C1 and A2 is connected to C2 in the relay. This will send the speaker signal from the original radio to a split connected to A1 and A2 so that both front speakers are powered.

The above assumes that it is OK to have the negative wires from the front speakers connected to both the original and modern radios at the same time and for the negative front speaker wires to be interconnected. I might be able to find a relay that has 4 internal switches, but that might be hard to find.