Well, as you can see in the picture I posted, I have the digital SPA series 2 gauges which have fully adjustable back lighting, in red and green light. So, you can adjust as you see fit. I don't know about analog gauges, but if a person wired the back light into the factory dash light wire, I would think they would be dimmed down by the factory dimmer switch?
Just an interesting bit of trivia that I wasn't aware of at first: with the SPA gauges, or at least my series 2's, the back light is *always* on, even in daytime, and even when the truck headlights are not on, nor the driving lights. How do I know this? Go in a tunnel and guess what? You see them lit up. Also, if you switch the back lighting to red, which the SPA series 1 gauges do not do, it is instantly obvious in daylight.
Tom