You can always fight it, but that doesn't mean you will win. However I still would take it to court, especially if it was a night an no other cars around to help judge speed. I'm not sure how long the light has been out, but if it just went out very recently (hint, hint) that might help weigh in your favor.
Your dashboard light was out - this is YOUR responsibility. If that is the only defense you have, you might consider traffic school (if that is an option in your state).
There are other ways to contest a speeding violation, but it will depend on the state you are in and the specific violation for which you were cited.