Weird stereo issue

Bought this truck used with 100k on it, drove it home, and the audio system seemed wacky, because the music only came from the front speaker in the dash.

So, the truck sat because I waited for parts from Toyota to do the axle boot replacements, brakes, rotors, etc, for at least a month before I drove it around. Magically, the next time I hopped in and actually drove it for a while, the stereo and speakers all worked just fine, and life went on, as the truck "healed itself" magically, when it sat for a few weeks.

So again, truck has been sitting due to my working out the P2714 code issue which although I believe still exists, I was busy doing other things. Havent driven her much.

Yesterday, wife and I get in, and about an hour from home, I turn on the radio, and the magical "non working" speakers has come back. If you try to adjust the speakers to the rear circuit through the main head, nothing comes from them. Move the adjuster back to center or front, and the "tinny" music plays.

Since "radio" search came up on this with little to go with, can you point me in a direction, or words to use to properly search this issue. I assume this is a short of some sort, most likely from an amp, or speaker line from head to the speakers in the back of the cab.

Thanks in advance.