If all the lights are bright, then the grounding for them is good. A poor ground to a light leaves that light dim compared to the others. A headlight with a poor ground will look orange in color due to having to hunt a ground.
So if they are all bright (high beam as well as low beam) and your tail lights are bright then I would have to lean towards the indicator being wired to the wrong terminal on the dimmer switch.
Is this a stock under the floor dimmer switch, or a later dimmer mounted inside the cab?
