1956 Mercury Monterrey 312, temp gauge just sits on C and never moves.  It rests at H when the ignition is off, but with ignition on it rolls over to C.  If I remove the wire from the sending unit while ignition is ON it also rolls back up to H, just like if I had turned off the car.  There is only one wire attached, but two terminals on top, which is a little weird.  I get a reading of about 20ohms between the terminal and the block  (car was warmed up at the time)

The sending unit looks just like this one but not sure if it's bad or if it's the wrong one.  I'm thinking about buying that replacement, the listing says for 1956 Ford but I know the Mercs can sometimes be a bit different.  At $60 I kinda want to make sure it's the right one first.