Dimmer switch causes gauges to peg max!?

Yeah,I think this model has a 5v dash gauge regulator.And it appears that your gauges are receiving full battery voltage, instead of the pulsed and averaged 5v signal from the regulator. I would disconnect the regulator and if the problem persists, then I would remove the cluster from the car and hunt for the cross feed.

It could also be that the dome lite wiring in the 8-pin connector behind the left kickpanel, is feeding power into the fuel gauge (which is connected to the temp gauge, thru it) sensor wire; they are side by side in that connector. There are other wires in that connector also that could be shorting. I would go here first,lol