We had another bout of rain here in Southern California last week. My (ghetto) electrical tape fix seemed to work. I can confidently say the leak was coming from the windshield seal and not the sunroof drain. The location on the inside of where it was wet also confirms my suspicion.

So today I used some black automotive silicone sealant and filled up the gap where the windshield meets the vehicle frame. Hopefully this permanently resolves my leak.

 

It's been a while, I was wondering if your sealant fix has been working alright? My W212 is suffering from an identical issue and I was also thinking of applying such a "fix" until it's time to replace the otherwise pristine windshield.

 

I know this thread it’s a bit old now but I just want to register that this indeed seems to be a factory problem. I live in Brazil and own a 2013 c180 coupe. I had noticed this leak a while ago while washing my car but neglected it. Recently, after a whole night of heavy rain, woke up to soaked headliner and passenger footwell. I found this thread and, voilà, the windshield seal has failed in the passenger side. I can push it from the inside and see the glass move.

I’ll try to have it professionally resealed and come back with the results.

 

Seems like something that can be done diy:

Not sure which specific MB sealant they use now.