I took one off a 2000 and their were threaded nuts that were attached to the underside of the roof (didn't figure out their way of attachment).
When I put on an aftermarket rack, I used sheetmetal screws. The problem with that is if you torque the screw too much, it pops the metal (because it is quite thin) and then you have to try and reform it. I used black silicone sealer on the underside of the rack to seal the holes and act as an adhesive to give it better holding power.
If I were to do it again, I would remove the headliner and JB weld nuts with a flange on them to the inside, and use bolts that go flush to the end of the nut.
Turtle installed a rack but I don't know how he attached it. Hopefully he will be able to answer this also.
Good luck and take pictures with however you decide to install it.
Everet <img src="/forums/images/graemlins/cheers.gif" alt="" /> <img src="/forums/images/graemlins/kewl.gif" alt="" />