ground issues and general wiring help

[ace347]

Here's the combo: I bought a 71 c20, mostly complete minus the engine. Over the past two years I've installed a 07 6.0l truck motor with the original granny 4 speed and replaced some of wiring if not tried to fix the rest. I almost have it road worthy (perhaps not technically road legal).

More specifics on wiring situation: The headlight harness is all new, because of farmer john butchery, courtesy of LCM. I took the engine harness off. I pulled out the dash harness and went through it, deeming it worthy of re-installation since there wasn't any splices or rats nest. The chassis wires that go back to the rear is original except for a patch or two. The tail light harness is new as-well. The gauge cluster is in-tact and I got all the gauges to work ('cept speedo cause cable and all). I am missing the wiper switch(plan to take care of that tho). all the bulbs, fuses and blinker switches are new and they all seem to work in some way shape or form. (no bulbs in rear markers yet). No fuel tank wiring(removed in-cab tank). No interior lights aside from dash lights.

The symptoms: When I turn on the running lights the passenger front side marker does not light up. With the lights on, the passenger tail lights works great but the driver tail light is dim and when you step on brake it turns off completely. When the lights are off the tail lights work as they are supposed to but the driver stop light is very dim. The blinkers do not work by them selves but the hazards work just fine, even the pass front marker blinks. The gauge lights don't work when the lights are on. The e-brake light comes on when cranking the motor but not when the e-brake is actually utilized. The turn indicators on the dash come on with-out blinking only when hazards are engaged. I have one mystery plug under the dash that I assume is to the missing wiper switch(three prong in the shape of a mostly flattened L). The back-up lights don't work yet 'cause the reverse switch pigtail goes through the engine harness.

I understand that there is a ground issue somewhere but I'm not sure were to start looking. I'm a novice at best when it comes to diagnosing these problems and a multi-meter is Greek to me. I found diagrams but can't figure out what I may have missed.

My question is if the main fuse box ground is on the engine harness that I excluded. I would appreciate any tips or tricks you can give me.