Actually the best way is to write a feedback email to Mercedes Benz in Germany. Surprisingly many MNC took complaints quite seriously, even their sole distributors here may not be as professional as their German counterpart.

I have never written to Mercedes but I did with my company car Sonata. I wrote to Hyundai Korea to complain about the a/c problem which were shunned by many SC here and surprisingly, I got a call to replace the compressor in about a month after that.

They got a feedback form at their website. I wrote my complaints there. You do need your chasis number.