I have an '83 P'up too, but mine is a 2wd LS model. I didn't know
parts were getting so hard to come by. About the only thing I replace
regularly is the rubber pad that goes on the brake and clutch pedals,
and my dealer orders those when not in stock.
I had a friend with a shop, and he had a wrecked P'up on his lot that
I occasionally got something from. I think the salvage yard may be
your best bet for things like a fuel cap. You may want to check local
alternator shops to see if they can rebuild yours, if you can't get a
reasonable price from the dealer. There are also some online stores
that claim to have parts.
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