I just did this for a friend on his 2002 ML320. His leak was in the same place. I replaced the O-ring (it is actually triangular in section, not round) and was advised to replace the reservoir as well, as there have been problems with them. I think that is actually what was leaking, it appeared that there was a small split in the area where the two halves are put together. It was an simple job, took about 20 minutes. There is a retaining clip at the back where you are seeing the leak - it pries out with a screwdriver. There is one bolt at the front. It is hard to get off due to the O-ring and there are two grooves molded into the reservoir that act to retain it to the aluminum piece that the bolt goes into. It is even harder to wiggle back in. The O-ring is about $1.40 and the reservoir was about $35 from a local parts dealer who sells MB genuine parts at 10% off.