I'm trying to wrap my head around this problem and I'm not sure what the answer is. Here's the run down:

Omori electrical boost and oil pressure gauges
They were wired incorrectly (not by me) so that gauge power was tapped into illumination. If I had the lights off, the gauges were off. If I had the lights on and moved the dimmer, the gauges would have less power (and get dimmer).

I partially fixed it today, by giving power to switched 12v. But I have a (much more minor) problem now.

The gauge functions with the lights off or on. If I move the dimmer, the gauge is acting the same as before (dimmer = less power to the gauge). Why is this?

My guess is its wired like this (I lost my multimeter, so if no one can confirm my hypothesis, I'll borrow one this weekend):

Power - 12V switched (ignition)
Illumination - 12V switched (illumination)
Ground - Variable resistanced ground (illumination)

Is that how the dimmer works in Subarus? Thats the only thing I can come up with..when the headlights are off the "dimmer ground" becomes a full ground, but when the headlights are on the resistance is varied (I'm guessing).

So to fix my problem, I'd just have to move the ground to a chassis ground, right?

Car is a 2000 Impreza with a 2004 WRX swap.

Subarus do indeed use a variable resistance to ground to control the lighting for the dash. So if you don't want the lights of the gauges to vary with the dimmer control and be full brightness at all times then you are correct about tieing return side of the gauge lights to chassis ground.

Subarus do indeed use a variable resistance to ground to control the lighting for the dash. So if you don't want the lights of the gauges to vary with the dimmer control and be full brightness at all times then you are correct about tieing return side of the gauge lights to chassis ground.

I think there is only a common ground in the gauge, for power and illumination. So I'm forced to have full brightness. Not that I ever use the dimmer anyway. :)

Thank you for confirming the variable resistance, I was pretty sure thats what I was seeing.