Ive had this problem since I got the car. It only seems to happen when I start the car. When you start the car the lights and dials are quite dim when you rev the engine they go back to normal, when the revs drop it dims again. Also when I put the indicators on they dim, and if I use the electric windows.
Since buying the car I haven't touched the electrics so its nothing Ive done, and the car had nothing that was non standard fitted to it. I wonder if I need a new alternator.