Well, that was an easy trace...Well...it's not a diesel.
And I knew everyone was losing sleep trying to figure out the mystery. Here's how the story turned out...
The new stereo showed up while I was washing it today.
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So, as the first step to installing the stereo, I pulled apart the dash, and as Dennis suggested, I planned on tracing the wires. Then I discovered, it goes nowhere!
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I guess the previous owner found an auxiliary switch and put it in with plans to wire something to it someday.
Thank you to everyone who took the time to think about it.