Do you want to recondition the leather or just clean it? If you're
looking to recondition, leatherique. To clean, get some lexol cleaner
and conditioner (carcareonline.com if you don't know where to buy
it...get some lexol cotton sponges when you buy this too, 4 sponges at
least), for the carpets steam clean them (if you don't own a steam
cleaner, I know some grocery stores rent rug doctor's), for the plastic
stuff and vinyl use vinylex (made by the lexol people).

Hi Maia,  had that problem myself in a second hand 190E.  the previous owners
apparently never cleaned under the carpets.  Particularly bad if you live
in areas with snow so that you can get standing water during parts of the
year.  In any case, do lift the molded mats (not the ones you are likely
to have on top of them) out of the car, and what I did was to leave them
lying in the sun for several days.  the closed cell foam likely has cracks
and absorbed moisture.

cheers