I take it that the TV's brightness is better when viewing other sources?
DVD players are $20 these days. I'd say buy a new one. Buy it from a store with a 2-week or longer no questions asked return policy. See if that helps. The problem could just be a bum DVD player.
Or you could just have a bum cable. Have you tried other cables?
How are you connecting the DVD player to the TV? Analog, digital, etc. And what kind of TV is it, and what kind of DVD player is it?
Torben