If one side is working normally, then on the side that flashes fast you have:

One light out.
Aftermarket bulbs that flow less current than factory spec.
A bad socket (that light will flash dimly or not at all).

Replacing front and rear bulbs with a factory-spec bulb will tell you if it was a bulb, if factory-spec bulbs do not work right, I'd blame a socket - the front sockets are known trouble. The contact between the "bulb carrier" and the little tab in the headlight housing tend to become pitted. I flowed a blob of solder on the tab on the bulb carrier adn it fixed mine. See, the tab wears down/burns away and contact is spotty at best. Building it up with molten solder seems like a good fix, at least, it has worked for me.

You should know that replacement sockets are available but do not search for a "Crossfire" part, the parts stores computers do not list the Crossfire as a 'fit' for the part in question. I can't remember what someone searched for but they found them in a parts store, on the shelf. I think they were for some other Mercedes or BMW car.