Great work on the tailgate! I couldn't tell what you had going on, with needing to polish the brushed chrome panel in post #23, but in your following installed pictures in post #24 I don't see anything wrong at all, with the brushed finish or the paint. You really did good.
On the rear slider.
I don't recall if I ever replaced individual glass panes (my CRS is really getting bad). The whole slider unit is normally installed, and I did a lot of them "back in the day", mainly aftermarket sliders.
I looked at some Ford shop manuals, and there ARE instructions for replacing individual panels for the OEM unit (included below, see the box).
You'll have to remove and reinstall the window unit; no big deal, and much easier with two people. The rear slider is held in with a rubber gasket. "If it were me" pulling a 31-year old truck rear window, I think I would pre-treat the rubber with a good rubber treatment (like ArmorAll, but I'm not up what's the best these days- suggestions perhaps from peanut gallery?).
I used to start pushing the gasket out in a upper corner. There's a tool called a Cotter Pin Remover (we called it a "nose picker") that works good for GENTLY hooking and pulling, and working the gasket.
Reinstallation, it's easiest to set the bottom in place, work the corners in, push down while you pull the cords, and then it zips right in. I used to use dish washing liquid (like Dawn) to lubricate the rubber; that helps a lot (for removal and reinstallation). Just take your time and go slow. If your gasket is questionable, or if you tear it, I would think that new gaskets should be readily available somewhere.
This is an '88 shop manual. The '89 was word-for-word identicle, but the '88 book was laid out better (on less pages).
Good luck!
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P.S.- That was a great suggestion by
@Uncle Gump, to start a new thread, on the work that you've done, and will do with this truck. A lot of us here love to see a nice old one saved and cared for. Keep up the good work!