Perfect timing for your post. I just recently rebuilt my entire harness for the headlights, gas sender, and rear tailights.
This is the entire rear harness. (One of those 4 pin weatherpack connectors was added by me for splicing in for a trailer connection)
The rear harness is only connected with a 4 pin weatherpack connector, bringing the 12v in for the 4 circuits. No grounds in this connector.
Now, lets talk about the grounds. That rear harness is loaded with factory splices. Most are for the brown wire, and the other larged spliced circuit is for the grounds. They are wrapped up in black duct tape inside the harness. I bet that most of us would be horrified to see just how pathetic these connections are by now, after years of corroison and wear and tear has set in. This is what the splices looked like in tailight gound circuit for my 86 K-10...
I would highly suggest you pull that rear harness and go through it. My main ground was through a ring terminal bolted up to the bed under the LR tailight housing.
I added redundant grounds, back to the frame and into the harness, about 3 all total out back! You can NEVER have too many gounds.
I also grounded the bed to the frame and jumped it to the filler neck with a 4 gauge cable.
Your fuel sender has a ground just running from the sender right to the frame.
It sure sounds like the factory ground to the frame is suspect.
Try jumping a cable from the bed to the frame, and then route a jumper from frame up to the battery (-). That should cover all the bases. If it still doesn't work, then your rear harness is in need of a look through.
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