Having finished up putting the stereo wiring in the RX7 back to stock, I went to put the radio back in and immediately realized that the DIN connector that goes to the amps had more pins than the socket. This makes sense, since there are L and R outputs on the radio and it is a single cable.
After scratching my head for a while (the factory service manual seems to totally gloss over the whole “there are two 6-pin sockets and a single 8-pin cable” thing), and searching the forums, it was revealed that, as suspected, there is a Y-cable which goes here. Naturally, I don’t have that.
I began to wonder - how do they have +12V, ground, and four speakers worth of signal “+” and “-” (which is 10 signals) in an 8-conductor cable? After not finding any usable information, I figured I’d just ring out the 6-pin plugs at the amps back to the 8-pin. It turns out (see below) that all the signal “-”s are bussed in-cable, as is the +12V and ground to the amps! This is strange, but it does mean that they only need 7 conductors for the formerly-10 signals.
Where does this leave me? It looks like (fortunately) the 6-pin connector to the amps matches up with the 6-pin sockets on the radio (which do have a pinout in the service manual). This means from the diagram I scribbled up below (yes the 8-pin is numbered backwards) I can make the missing Y-cable.
The downside is that I had to order the connectors (of all the crap I have laying around, 8- and 6-pin DIN plugs are not included) so the whole “put the stereo back together/in” thing will be delayed until at least Monday.