There IS NO frame/ body in Mopars, they are a welded unibody. You are simply making a poor connection. If the gauge thuds to full when the wire is grounded, either you.......
have a broken connector at the end of the wire and it is intermittent
or the sender is bad
or the tank/ sender is not grounded. Hook up the sender "normal" Connect a good jumper clip to the fuel outlet nipple, and make DAMN sure the other end is grounded. If the gauge works you have found it. If not, replace the sender
