If you establish 100% that it is in fact your evaporator, the evaporator will simply have to be replaced with a new one. As others may have suggested, be sure to replace with an all-aluminum evap. Do a search to see how involved it is for your model - I know on my 300E it is a 20 hour job and costs about $3,000 if you have it done, but only a fraction of that if you DIY.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280