I can't speak for certain on a '99, but on my '98 the roof is a two-piece assembly, with the inner piece being somewhat of a "skeleton" piece. If you drop the headliner a bit in the front, you should be able to see the inner piece with its pre-cut holes in the locations where the wires would go for the cab roof lights. My '98 factory service manual supplement "Dimensions, Joint, and Seams" gives an exploded view of all the body panels. Hope this helps.
CTD12V