'Doc:

Insert or not, there are still two intake holes. The insert simply makes the fender hole smaller in area (and less efficient).

Can you hook me up with some more information? I can't see how that insert reduces the likelihood of ingesting water during a deep-water ford...

I also realize that the M-Class is theoretically capable of doing a bit of off-roading, but I've yet to see anyone take one out stump-jumping like I used to with my father's GMC (rest its poor soul) back in the day.