I spray some kind of heavy duty cleaner, or oxy-clean on the bad spots, then spray with a pressure washer. Hang up to dry (over a fence, cloths line, or something like that.
The pressure washer gets all the way down to bottom of the nap and cleans out all the dirt and grime. I have been doing this on all my cars for years and have never seen any advers effects.

__________________
84 500 SEL (307,xxx miles)