I've wrecked a couple of those plastic barriers. I didn't think they added much protection from moisture to the door but they seem to be there to prevent the cardboard panel from absorbing the moisture which gets past the window 'sweep,' which remove water from the glass. I stored a panel for several months in my garage and found it deformed with a huge bow which made it a two man job to reinstall.

Let's be honest, moisture is going to find it's way into the doors if nothing else but heat transfer through the drain holes in the bottom of the door. The idea is that none of that moisture softens and deforms the cardboard backer that holds the panel together. MB may have also intended that none of the cardboard flake off and plug the drains too.

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84 300SD
85 380SE
83 528e
95 318ic