Not a bad question....

I have a '91 tailgate (flush glass) to go onto my '86. The biggist problem I see so far is that the washer nozzle is top dead center on the newer t-gate, but aside the gate on the older car. That measn that (a) I must drill a hole and install a grommet for the squirter hose to go from up near the right hinge over to the t-gate, and (b) I'll have the original squirter just sitting there. (Maybe I can plumb them both in for a double douche.)

I am assuming that the key lock barrel and assembly will transfer from my '86 t-gate to the '91 gate.

You can solve the rust problem by opening the crimped-over sheet metal along the bottom (and also of the regular doors, too), scraping out the rust 'n crud, painting with anti-rust paint (I like Corroless), folding the crimp back while the paint is wet, painting again. Let it dry several days and paint with body color (or black).

The dripping water you see might be caught up in the weather seal, not inside the door. Shouldn't be getting in the door, anyway.
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Don Foster (near Cape Cod, MA)