Chevy C20 fuel gauge not working! HELP!

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I have been working on mechanical stuff on this beast and now I'm tackling the electrical. The fuel gauge reads almost full with key on AND off! Occasionally, the needle will move to over full when turned on. I have checked the sender (float, etc.) - it works fine. However, it doesn't have a ground wire... ?? Retraced the tan wire that supposedly runs to the fuse panel - no cracks or breaks. Here is part of my dilemma: when the tan wire runs up toward the fuse panel, it doesn't plug into it. It goes into a wiring harness/cluster. What gives?