lights behind speedometer are dull

sound like ground problem

Your instrument cluster does have bulbs that illuminate it. It will need to be removed and the bulbs replaced.

Hi, The instrument cluster light bulbs occasionally go out on the 03-06 Yukons and they can be replaced. However, they are soldered on the board and aren't exactly "easy" to replace. You can have your cluster rebuilt/refurbished including all lighting at www.drspeedometer.com -V8Turbo