Thank you Gilly for the info as to what I was dealing with. My Bose head and 6 disk rear changer could be a CM2294 but it actually says Matsu****a (Panasonic) CQ-LP2610Z, A003 820 74 86. Easy access was had by prying out the plastic storage bin under the radio to reveal a second DIN compartment for the Best/Insignia HD radio/single CD addition.

I split the existing antenna with good fidelity but no signal on HD-AM which I didn't want anyway. You can screw a short whip (6mm socket) into the telemergency/cell socket on the back roof for a good (it works), if conspicuous 2nd antenna (the coax is factory installed) or experiment with better splitters.

Facing the lower plug, bottom row connections are ground, illumination, battery,antenna(this powers up a rf amplier in the hatchback grabbing signals out of the defroster wiring). Lower plug top row connections are: ignition and, I'm guessing, power amp.
Middle plug is high level pre-amp out to power amp with bottom row all grounds and upper row:back, front, front, back.
Upper plug is CD changer and the signals are not useful, being in digital or Japanese or something.

I once faced the high cost of replacing the changer before I cleaned it with industrial alcohol and Q-tips. Instead, I believe you could replace the head and still use the power amp by using speaker outputs attenuated by an adjustable device intended for that purpose sold by Schucks. The amp probably draws some current and you might consider a power relay rather than direct connection to the antenna out of the new head.

I tried direct connection of the middle plug rears to my preamp outs but my level was too low and the amp compensated by amplifying engine noise.

Having spent too much time on the project and wanting 2 working heads I opted for a Audiovox RF modulator which gave me a good result.