It is located underneath the passenger side dashboard in the footwell area.
First, remove the plastic cover under the dash/airbag/glovebox area. You will need a regular socket as well as a phillips screwdriver. There is one wire for the auxiliary power port that you might want to disconnect, but I did not need to do this.

Once you swing that plastic cover out of the way, there will be a thin, black plastic trap door (about 2" x 10") that you can release via a tab and remove from under the dash area.

When you do this the filter should fall straight down, or you might need a pair of pliers to gently pull it out. You'd be surprised at the amount of crap that filter stops - well worth replacing. I used a MANN filter, which I think is the best of breed. Good luck.

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Gil

2004 ML350
1984 300CD; Ivory (sold)