I ran into this with an autometer c2 trans temp gauge.
iirc the factory instruments are dimmed from the (-) side and will not dim the led of am gauges.
To make it work for me I installed it on the hump by the transfer case shifter so the light wouldn't bug me.
Autometer has an add-on dimmer you can set and leave as desired bright/dim, or you can get a (I think) 1/4 watt 475 ohm resistor and wire it inline on the gauge light (+) power supply.