It's on top of the heater box. If I remember right it is under a "L" shaped connector and is held in by a couple of screws. You may want to take out the glove box for better axcess.
Have you checked the switch or possibly a ground?
Good luck.
I'm installing a Auto Meter water temp gauge on a 318 with an airgap intake. There are not any predrilled holes that are big enough in the intake for the sensor (1/2"). Is there anywhere else that I can get into the water jacket?
Fastback 340
Can you provide more specific info on the trim plate?
Outside diameter?
Gauge of the steel?
Gasket material?
By the way it looks great and looks to be a simple fix for the problem that many of us have.