Alpine do a unit which enables steerings controls on facelift w210 & w208, perfect for a retrofit project, i may be able to source one if you need one.
I found something for the W208, but it said it needed a 39 series adapter.... which was not listed below.
I contacted incartec who sent me two replies with varying prices (!) between ~90 and ~110 total including postage for the main converter and '39 series' adapter for your given HU manufacturer.
Suggest you give them a ring or email....they seem happy to help.
Word of warning...as a pre-test before spending ££££££ on an all-singing-all-dancing HU, I tested an old one I have. It had power (would eject CDs fine), but would not switch on. I think this is due to no switched ignition cable being present on the Merc's ISO connector. This is usually a red cable on ISO harnesses. So, in a nutshell, I think you might need to route this from the back of the cigarette lighter (as this only has power when the key is in and on position 2, or engine is on). I'm not sure though, so don't go doing it on my say so!