[CKfan]

Hooked the sending unit wire back up, the battery, turned the key to on and quickly pegged the needle again. Reversed what I had done but this time touching the back of the terminal and tapping on it had no effect. Still pegged.

Bad ground seems likely but everything else on the cluster works.
I have two ground straps to the cab already, as I know to use lots of grounding points on these trucks. Because the dome light doesn't work also, I will take off and re-tighten the ground straps to see what happens.