I was Service Mgr. at a M'Benz/Volvo/Mazda Dealer in the late 80's when these MX5's were new and many were in for warranty tops because of rear window zipper failure..
If they don't know this already, make sure to tell your friends to always undo the front top clamps and let the top open slightly before zipping the rear zipper then clamp the front last after the zipper has been opened/closed..
~ Pete