See the link on the Subaru McIntosh info that I posted earlier for descriptions of the harnesses.
You can re-use your 14-pin harness for power and illumination into the HU, but you'll need to disconnect the speaker wires and extend those down to the other harness under the seat. And yes, that is the one I can provide. It's either another 14-pin similar to the one for the HU, or an 18-pin (from the same connector family). No pictures needed, unless you find yours is something different.
Your JVC doesn't have RCA line outs for the 4 main speakers? If that's the case, then to keep the Mc amp you would need to use a Line Output Converter (LOC) to convert those speaker outputs to line level, and then feed those into the Mc amp's DIN cable. Ideally the Mc amp wants balanced signals, but people have connected unbalanced line level and it still worked. Basically, you just earth the negative inputs. But it would be easier to lose the Mc amp.
The Mc amp takes 4 inputs -- 4 main speakers -- and generates 5 outputs (4 speaker channels plus sub). The car connects each of the main outputs to several speakers and tweeters.
The plane is a Beechcraft Model 18 (AKA "Beech 18" or "Twin Beech"). That type happens to be one of the first planes I jumped out of.

The photo is from the US 1996 SVX brochure.