the old way ! drastic but it worked
I use a 1989 190E as a tacsi and used to take the seats out soak the with warm, water sprinkle some washing powder on scrub with a scrubing brush, wash of with a hose pipe, suck with a vax wet'n'dry and alow to dry in the sunshine !
VERY DRASTIC ! I know but cleaned them at least 20 times this way and always looked like new afterwards !
got a c180 w202 now and wouldnt dream of doing that to the seats for one reason the cloth just wouldnt take that sort of scrubing as with everything else in comparison to the two models the c class isnt built with half the quality.
so at the moment I just use the vax with 1001 cleaner.