Auto Parts stores like AutoZone sell a leak detector kit that has a flourescent dye and a miniature UV lamp so you can do your own leak detection. You install the dye into the AC system, and assuming you already have enough refrigerant remining to provide adequate pressure to make the compressor run, using the lamp will show you a glow where the dye is escaping...
Also, you are better off finding an independent Mercedes mechanic than trying to find an AC shop that will service Mercedes.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."