... I color-coordinate the heat shrink.

Yes, I used high temp 600V hookup wire. It’s the only remotely-appropriate wire I had on hand.

This evening I made the Y-harness I was missing for the stock stereo in the RX7. It goes from the 8-pin DIN plug in the harness to the two 6-pin on the back of the radio. This would have been in the car at some point, but whoever put in an aftermarket unit then hacked up the wiring attempting to put it back to stock (presumably before selling it) lost it. Tomorrow I’ll plug the end into the harness and verify I have continuity to the amps as I expect (aka make sure I wired it right), then finally put the system back together and into the car and find out if it works. It didn’t when I removed it, so it shouldn’t be any worse at least...

The +12V and Ground in the Front plug are bussed with the same in the Rear, which makes for only two wires to the one. This was good news for me on the 8-pin end; these are not exactly easy connectors to solder to.

Of course, I’m already in the process of looking for the higher-end factory system from a GSL/GSL-SE with better amps, a digital radio tuner, and graphic EQ (!). Fortunately that uses the same harness as the base model setup.