Bad Key FOB?
When I had my M5, my fob worked exactly the same way with almost no range. The Comfort Access (same as Keyless Go) was spotty to say the least. It turned out that the Diversity Antenna, which was located below and connected to the antenna fin was shot. I replaced that and my range increased to over 150 ft. Perhaps your Benz needs the same kind of part.
Low voltage was causing my key fob problems (I could not lock/unlock the doors or open the trunk with the key fob). I changed the supply battery (in my trunk) and all of my symptoms disappeared.
2005 Acura RL, 2007 Lexus LS460L, and this 2011 Mercedes S63 AMG.
When the driver door handle gets wet it stops unlocking itself when you grab the handle. I have to grab the rear door handle to unlock all doors.






