67 cuda instrument cluster

One common reason for the temp gauge not reading is the failure of the instrument voltage regulator. This is a solid state unit built into the fuel gauge (odd, but true). You can buy a unit to replace it, but it has to be soldered in.

You can remove, or just tilt out the instrument cluster to access the back of the gauges, but there are a lot of connections. I would recommend following the procedure in the factory service manual, so as not to miss a step and possibly damage something. The pins in the big circular connector are particularly fragile.

Oh, just because you don't see a wire attached to those pins doesn't mean they aren't wired. That blue board is a printed circuit board, and the gold traces on it are the instrument "wires". So that's another fragile component.