It would not hurt at all to put in a new battery. Two years is about the max I have been able to keep a factory battery on my cars. I always switch to Sears sealed batteries, they last over five years.
Although, a quick way to check if the battery is an issue, you shouldn't have any problems opening the roof with the engine running (the alternator would cover for the weak battery).
I don't know how "friendly" is your M.B. dealer in your area, but mine always let me switch the suspected relay with a new one as a quick loaner. If the problem is fix, they just invoice it and everyone is happy.