Does the light fail to come on when either front door is opened, or just one door? If it works OK with either the passenger or driver's side door, then its the door switch on the 'bad' door. Make doubly sure you have the three position switch at the dome light in the correct position. If its set to "off" by accident, the lights won't come on with either door.

-Mark- '99 Mystique LS, totally original (including the original water pump and the premium stereo)! Bought new in December 1998 for $21,000. 208,000 miles