You can download a copy of the Owner's Manual for your 1999 Ford Explorer at the following Ford Owner's link: https://owner.ford.com/tools/account/how-tos/owner-manuals.html
Hopefully, you get the information you need about replacing your interior dome light switch there. It's usually located somewhere around the interior of the doorjamb. We've seen some information that indicates it can be a broken or faulty ground wire.
I also found this video on YouTube which might be helpful for you: https://www.youtube.com/watch?v=FqxukQvO9Lw
If that doesn't help, please find an AskPatty Certified Repair Shop at http://www.askpatty.com/search/?type=all&m=30&q=&keyword= and they can help you swap out your switch.