I removed part about gauge being grounded. Now don't think it should be. Also rewrote some sections of my post. Sorry.
Sometimes a light compatible sender is used by mistake on a gauge circuit and may cause trouble. If sender has a single terminal, it is most likely a gauge sender. Two terminals, it's a light/annunciator sender. What motor is in the 66?
You say you took wire off of sender and touched that wire to a good ground on the motor or ran it over to negative battery , with key on, and gauge did not move off of cold. Is that correct?
You keep saying grounded "sender circuit", just want to be sure you grounded the loose wire to a ground spot. Key on. Motor does not need to be running. Just being sure our words are the same meaning and that test was done according to Hoyle/as Wrenchbender said.
Somehow you will have to perform the tests described in previous posts if grounding sender wire at motor end has no results.
As far as amp gauge, when key is first on you may see a jump. With motor on, it should go to middle or slight pos. When near middle, turn on high beams, should move toward minus and recover toward pos. Approximately. Rev may produce small movement to pos.
