The late model YJs have dimples in the sheetmetal where the insta-trunk screws go, unfortunately, a '91 probably doesn't have them! The original hardware kit has #10-24 "wellnuts" that require 3/8" holes to be drilled into the side panels, and thumb screws with large washers to secure the insta-trunk to the body. You could use self tapping screws to secure it, but if you are going to remove the insta-trunk often, the wellnuts are a better option. You should be able to find them at a better hardware store. Either way, when you drill the holes, don't go too deep, or you will drill through the plastic inner fenderwell liners that help protect the body from rust. Hope this helps, good luck!
