FWIW ;
I always use original Mercedes window grommets, they're available from The Classic Center and they FIT and DON'T LEAK , period .
They'll discount them slightly , enough to where it's like getting free shipping .
3M makes a very good urethane window sealant , I'm in The Auto Trade and have been using it for 40 years , no problems ever .
However , taking the time to properly clean the pinchweld whilst the old glass is out , helps prevent leaks too .
I rarely have to use the polyurethane stuff (you inject it using a
tiny nozzle on a caulking gun) unless the unibody has suffered serious collision damage .
The rear window (' backlight ') of my '82 240D was giving me fits , leaking , always leaking until last month when I replaced the original 18 gallon fuel tank with a 21 gallon one and discovered the ding danged drain hose in the fuel filler recess , wasn't properly attached in spite of having the proper clamp and good rubber etc.
I re connected it and haven't had any water in the trunk since .
Less Diesel fuel smell in the interior too
.