The issue with the tailight seals lies with the installation which was done incorrectly from factory. The adhesive part of the seal should be placed on the chassis of the car and not on the tailight. I replaced both seals on my car as a preventative maintenance and installed genuine MB seals which werent that expensive. Also installed two covers which I found in MB EPC software and it has been dry for three years now. About placing the rear SAM in a plastic bag I would be worried a bit since it may generate heat and be unable to dissipate that heat which may lead to other issues.
Your installation is incorrect. The seal is glued to the body and not the tail light. It will eventually leak again. Take it apart and fix it correctly.
