Some questions - of course:
Who is "we"? the Dealer/MB and you, or you and your mechanic?
Why do they have to be redone again? Same problem or different problem?
OK, the 'we' is basically my wife and I. I purchased the two replacement tail lights from an online MB dealer I've used several times now.
So I guess you could add that MB dealer into that 'we'.
But since I turned 60, the wife doesn't require ME to do the work any more. Our mechanic is super, mainly working on my stuff (chevys & dodges) but he has also done work on HER SLK32 AMG as well. He replaced BOTH tail lights in a cold light rain while the car was under the awning in less than ten minutes and then tried to wave his fee, which I had to put my foot down.
I pull my own freight OR pay for it to be pulled these days, anyway.
So I guess that the 'we' is all four. ME for ordering the parts and knowing which dealer to use, My wife for owning the vehicle, the online MB dealer for the part and my mechanic who first discovered the issue and then put in the replacement parts that I got for him.
I hope that helps.