As previously noted, the harness that came with the radio had headset wires that were far too short to reach to the back seat area where mine are. So I had to splice the harness into the wires I had previously connected to my jacks.
I butt-spliced each connection, holding them in place with solder, and then putting heat shrink on each connector. Then another heat shrink to capter all the wires.
I tried butt splicing with crimp connectors, but these are 22AWG wires, and the crimp connectors often did not hold the wires, even when I clamped down hard with pliers after crimping (you're not supposed to use pliers on crimps, but the crimpers weren't doing it, so I tried the pliers).
Connections in progress. Butt splices are soldered and then heat shrunk.