I just fixed a similar problem on my 1982 280e. If you are lucky, it is just the potentiometer that dims the light. You can remove it and replace it or, to save money, you can just bridge it by soldering a wire between the terminals where it plugs in behind the dash. In my case, I also had to bridge over various parts of the printed circuit that had somehow mysteriously been worn off the back of the dash. In any event, it was a pretty simple repair that took less than an hour (once I figured out the problem)