I was still typing, re read in case you missed anything. Both the relay AND the horn grounds must have very good grounds. All other grounds must be bare metal attachments. The only "return" for the electrics is the WHOLE truck. Frame, body, motor.............to the battery. VERY critical. This is a long and torturous path. Everything is rubber padded/mounted. So braided cable, or other, must be used to bypass all those rubber isolators.
I.E. Frame to body, body to motor, motor to battery........in any order. Not to painted surfaces. Flexible braided cable is best.
Looks like you are doing a really good job and a very nice truck is emerging.
I hope you cleaned out that column hub, where the horn parts go. Can't get a ground thru that gunk.
