Rear lights wiring harness issue

[HIGHWAY BY THE SEA]

This electrical issue still has not been fixed yet due to me concentrating on other things with this truck. It is coming along overall well in other areas, but I cannot believe how many electrical problems that have popped up.
I lost all dash lights after I got them working so well (even the heater switch light), the horn never worked with the horn button; only with a "jumper", and now it does not even work with that. Still no signal lights, brake lights, inner-cab light, or license plate light. At least I got the cigarette lighter and plug fully operational so that I can charge my cell while working on it outside.
I am SERIOUSLY considering replacing the entire fuse box since there is visible rust on the metal contacts. The wires all seem OK though. I have just about decided to have a professional mechanic fix this issue, because I have gotten NOWHERE, and am sick and tired of dealing with it.