some people on the sirius forum (I think that's where I saw it) have been making their own adaptors out of old scrap computer led and power wires. You cannot use a cd changer with these, but they are just about free if you know what you are doing....this concept might apply to the mercedes radio as well. I'll look up the link later if you like.
1995 E420 - 106K - Brilliant Silver Metallic/Orion Gray
1984 300SD - 151K Manganese Brown Metallic/Mostly Black
1982 240D - 79K - Manila Beige/Palomino---Sold