I put two Autometer gauges in my Ram today, a boost gauge and stepper motor type fuel pressure. I tied the lights into the headlight/dimmer switch, just like I did for the Air Lift gauge. The weird thing is, the lights come on fine on both the new gauges, but the dimmer doesn't work for the boost gauge.

I double checked the wiring and have both light wires tied together going into the headlight switch wire. Both are also tied together to a common ground. This is really bizarre and I have no idea what to look for. Anyone ever have this problem?

Is the boost gauge LED light?

Most LED will work on a range of voltage

Are the gauges the LED lit ones or the regular lamp ones?

LEDs can be flaky when it comes to dimming them...

Yes, they are both LED, as well as the Air Lift gauge light. Well, I assume they are LED as they are internal and cannot be removed like the old style.

It has two male spades out the back, one marked 12v. I just went out and reversed them just for grins, and got not light at all on the boost. Flipped them back where they belong and it still works, but no dimming.

Now that it's dark out, I went out to see how bright they are and they are BRIGHT. I think even dimmed down, the fuel pressure gauge is brighter than the factory gauges at full bright. So, I imagine I will just live with it. Since they are on the steering column, I will just throw a rag over them if they are too bright at night.

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.

How about trying some window tint on them?