I have the same chassis and according to the operators man., we are supposed to have a low speed fan selection in the defrost mode, but mine also does not.
I believe it is a typo in the man. as I checked with another E320 owner and his is the same.
Or we all have the same fault.
PS -- The man also calls for a heat dependent variable speed in the defrost/auto mode, but again I only have high in this setting.