I think your truck was a disco in its previous life.

You might try sandpapering the ground wire on the firewall, make sure all those contacts are good. There's a couple key ground wires that need to be well-connected (there's one on the trans, too) so take your time and just check all the ground wires.

Someone else on the forum was having a similar problem, and hopefully that person will see your post, and perhaps give you better direction. Meanwhile, don't play any Abba or Bee Gees or the truck might produce a mirror ball, and a John Travolta bobble-head doll appear on your dash.