i take them out, and i spray a degreaser on them and hit them with a pressure washer....ive been detailing for years and this is the way i clean floor mats:

1.spray degreaser liberally on floor mats(like simple green). Allow to sit for a minute or so.

2.pressure wash mats 1 at a time on a medium pressure. wash pushing all of the dirt/excess water forward and to the sides.(you will see all of the dirt/mud coming off) you do not want to miss a spot(i think of it similar to cutting grass.)

3. use an ice scraper to squigee off excess water, and hang mats up to air dry.

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95 E320 24k
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2013 SL 600


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