Sounds like the plug contacts got corroded while the truck was stuck in the barn, Grouch.
Some plugs can be dismantled and cleaned or it might be possible to get replacement
plugs and wire them in. The last thing one wants it to have lights go out on a twisting,
winding road on a pitch black night. Another option would be to cut the dodgy plug off
and fit insulated female disconnects to the wires allowing direct connection to the bulb
tabs (Marking each wire's function as it is removed from the plug

)
Ken.
Stripped nuts, as you described, can be a right pain in the butt to remove. Ideally I
used to use a nut cracker - given room to fit it. Sometimes it was a ball breaking
job of cutting it off by hand with a four or five in length of hacksaw blade. I don't
know how much room there is on your truck but could you tack weld another nut
to the stripped one then use a bolt through the welded on nut to 'jack' the stripped
nut off the stud?
FWIW.
Ken.