Trying to figure out why my fuel gauge doesn't read correctly

[HeavyHauler]

On my 1968 Chevy C10 the fuel sending unit had already been replaced with a new one by the previous owner but that did not fix the problem of the fuel gauge not reading correctly. The gauge reads empty with the key on however when you turn the key on and then remove the wire to the sending unit the needle for the fuel gauge instantly moves to the 3 oclock posiition and stays there but when you put the wire back on the needle then goes right back to the empty fuel location.

Anybody have any ideas on what could cause this?