I'm surprised Mercedes didn't give you a detailed printout of the STAR session.
I don't think the age of the car makes that much difference - my A200 is also 2006-reg and the local dealership is always happy to oblige (it is on the Service Care Plan, in all fairness).
Steve, are you in the UK by chance? I might be able to direct you to a trustworthy place, to look into this problem.
Yes, the drains are by the wipers, but also check the one under the combination filter - let me know if you need a guide on how to remove it, but I think it's described in full on lofty's page. Found it: Bert Rowe, Lofty's homepage. A-class info W169, Activated carbon filter, Location Replace,
Re: CD changer - as I've just installed an aftermarket head unit, I have removed my CDC, MH3511 MP3-capable, in perfect condition and working order - so again, if you're in the UK, we can arrange something. I also have an Audio-20, that can be used to test your system, as I know it works perfectly. I'm in Northampton.
Yes, the STAR diagnostic will pinpoint any major sector on SAM, if it returns an error (as far as I know).
One question - does your CDC's buttons lit up when you open the glove box, right after unlocking the car (with no ignition on yet)? If not, then it might be the actuation trigger is malfunctioning - same error would shut down the head unit, pretty much as you described.