How can I fix the door switch for interior light?

Does the one on the passenger door work? If so it could be something as simple as corrosion / dirt on the switch contacts in the door pillar. Under the screw holding the switch in place, pull it gently out of its locating hole, operate it a few times by pressing the plunger, spray the contacts with WD40, check the spade terminals are firmly on and clean. Replace the switch in place, do up the screw and it should work hopefully.
If neither side works it could be the interior light switch is on the wrong setting, but you don't say what car it is.

Chicks Fan
December 2008