My 70 Challenger didn't originally come with them, but I found a set of factory louvers. The problem I'm having is the two center ribs (the pieces the slats are attached to) on the louvers hit the glass, and I don't seem to have any adjustments to make them fit. I noticed the rear glass has a curve (top to bottom). The ribs hit on the high points of the window curve. The holddowns can't be raised any higher-no more threads left. Without the glass in the car, the louvers fit perfectly. I'm assuming cars with louvers didn't have a different rear window from non-louver cars. Any ideas?