OK, the PO obviously installed his stereo himself, and instead of getting a wiring harness kit, he cut the factory harness off. And he didn't have the rear speakers wired up up, but even after I did, they didn't work. So I brought out my home stereo speaker to test, and it worked so I got new speakers.
And the wire color sticky thread is wrong, at least for my truck. I got the correct colors from Crutchfield. Everything is all good. Got a new unit and rear speakers today, and it works perfectly. It feels good knowing it's done right.