56 bel air temperature gage

all original instruments.
when i took delivery of my 56 i noticed that the temp gage would stay at cold and then when the motor warmed up it would jump to boiling. no inbetweenie.
used a laser temp gage and found the top of radiator was running around 180. so it was not boiling.
also drove the car for some miles without problems.
bought a new sender unit from one of the suppliers.
same thing happening.
you smart folks here were able to tell me that the sender unit is for 56 only. just a one year thing and other years will not work.
one of you nice people sold me 2 used 56 sender units and checked them before sending them to me. i trust in you and dont think its your sender unit anymore. heck, iv been through 4 of them now.
so..............
tried looking at the wiring diagram in the shop manual.
looks like the wire from the sender unit goes straight to the gage.
the diagram does not show where the power comes from, so im going to assume the power supply goes to the gage and then to the sender unit and is going to ground at the sender?
as the gage moves it appears im getting power to the gage?
does the gage run on 12volts or is there a step down before the power gets to the gage?
any other thoughts on what my problem is?

thanks
Frank