To paraphrase the late Hunter Thompson, "things just get interestinger and interestinger", as I proceed on into this install.
As most of you may know, I bought the Yukon last summer from a seller in Utah. While I asked questions about condition, I never asked about the audio system.
It turns out that one of the previous owners had installed a single DIN Pioneer DEH-P8400BH, which isn't a bad head unit at all. I removed it this morning and instantly found out why there was no sound coming from it. The installer had connected it via all the RCA connectors to an external amp and subwoofer. I'd suspected as much because there's still a huge power lead coming from the battery and running all the way to just behind the rear seat. Not much other use for a 1/4 inch connecting wire.
I disconnected the RCA lines, and the rest of the plugged in leads and now have the head unit sitting on my desk. Much to my surprise, the main head unit connector is the same on the DEH-P8400 as on the new AVIC unit I'm installing. No Sirius/XM port and no navigation antenna port, but it has all the other "stuff".
That means I'm unlikely to need to splice in a new adapter cable since that's already been done. I'll just need to place the Sirius/XM receiver, connect it, and place the antenna for that. I'll place the nav antenna, and I'll be done with the wiring.
I bought a double DIN bezel, just needs paint to match the tan interior and I'm finished. Photos later.