I think you are on the right track by doing the process of elimination.
Either the wire that feeds the lights and horn on the fuse block isn't hooked up or there is a bad connection in that circuit.
Will the lights work if you turn the key to Accessory?
Pull out the instruction sheet and see if you can figure out how the fuse panel for the lights is supposed to get it's power and backtrack.
This is supposed to be the stock wiring diagram for that truck.
http://chevy.oldcarmanualproject.com...df/55truck.pdf