There are two filters, the last issue of the MB"star" mag did an in depth illustration. The "activated charcoal resides in the heater cavity, and requires the removal of the windshield wiper /transmission /linkage assembly. I think the dealer looks at 3hrs or so and a few hundred in filters. I am pretty handy, the scope of the job still scares me a little, get a copy of the article. I think it can be accessed on the web.
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