The only problem with doing a variable dimmer is that different cars control the dimmer differently. Some do it with voltage, others do it through pulse modulation.

Once upon a time, Hugo expressed his desire to eventually implement variable dimmer, but he probably hasn't gotten back to that one yet.

Personally, for me, it would be a useless feature. I never change the brightness of my dash lights, they always stay at the same brightness. So once I've set the player's brightness, there is no need for it to be variable.