The problem: Cracks appear under the Wing Window. Crank the window down; remove the window securing bolts, and the slide. Grease the slide and lay it aside. The grease makes window movement much easier when you put everything back together. The Situation: Remove a sill from a donor vehicle (This is a Mitsubishi Motero Door). Lots of doors are junked at the recycle yard every day. You should pick one that has a sill 34" in length and with the same radius as the CJ sill. They will NOT be hard to find. Have the Recycle Yard just burn it off the door, you can clean the piece up later. I used a cut-off tool and an air chisel. It took about 15 minutes to remove the piece Remove all the paint, primer and the filler from around the crack. You must be down to base metal on each side of the crack.  Spot weld up both sides, and finally up the middle of the crack to join the sides.  Grind flush, an seal/paint weld to keep rust from forming. Strip the CJ Door of the pull handle, the window crank and the decorative trim and the screws at the top of the frame that holds the top of the window slide. These pieces need to be removed for access. Lay them aside. Punch 1/8" holes into the donor sill.  Place the   sill over the widow area and start by spot welding the top to the CJ doorframe. Then tack the bottom of the sill to the to the CJ frame working from the middle out to each end. When finished, grind down all welds flush with the surrounding material. Remove the securing bolt that holds the bottom of the window slide. Let the window drop all the way inside the door. You can remove it in a couple of steps. This is the top view, looking down on the sill, under the Wing Window area. Note the holes to spot weld through and that I cut it out to drop the sill down into the existing window slot.