If you've gone to LEDs, the corrosion, or extra resistance, wont play as big of a factor, this is due to the fact the LEDs are constant current devices, and not voltage dependent. This is why LEDs are not dimmable in the traditional sense, varying the resistance, doesn't affect the overall current supply through the loop, just the voltage. LEDs require a PWM Supply to be dimmable, and then the LED isn't truly being dimmed, just turned on and off very quickly so that it appears to be putting out less light.
If you don't have LEDs, then yes, every bit of extra resistance is going to drive the overall voltage for the circuit down. And this is everything from the rehostat all the way to the flex circuit. You can dress it quite a bit and get the resistance down to virtually nothing (got my car to about 3ohms) but the lights will still be very dim.
You can remove the lenses that are over the bulbs, they're pretty easy to take out once you open up the back of the instrument cluster. But be warned, the little fingers that hold them in are most likely incredibly brittle, and you'll more than likely break them off in getting them out.
I took one out, to see what kind of a difference it made in brightness, and I wasn't impressed. I put it back in, and super glued the fingers back in place.
The problem stems from the design of the cluster itself, the lights are located behind the gauge faces (which are stamped steel with no windows for the white lettering) and the light has to bounce around the back of the gauge and make it through the quarter inch or so gap between the face of the gauge and the decorative plastic ring on the inside of the cluster.
So at best you get very indirect light, which leads to an overall "dim" feeling and appearance.
I'm currently investigating what the best way is to get the most POP out of my gauge lights. Simply switching to LEDs didn't do it for me, the light is a little brighter, and a little "whiter" because of the 5K leds vs the about 3.7k bulbs.
Hope this answers your questions!
