1988 C3500 1ton 7.4 No Start Problem

[Gpur58]

Gpur58. 1988 C3500 7.4 1 ton no start problem , rebuild engine put back in . lost power to left side of fuse box which runs ECM , Head lights, jump that side of fuse box and it started, just like like hot wiring I guess, but still can't find out the problem to no power to left side of fuse box, traced wiring , replaced relays , switches , at a loss any help or ideas would be great. What a fantastic sight this is. thank you