I am with arthur on this. The drive authorization on these is a pretty common problem. Quick question, do you have the red and green lights in the mirror and doors or do you have a large ring around the ignition where the key goes in? You should have the ring, I think. Mercedes used 3 different drive auth. systems from 1996-1997, before going to the new smart key systems. If it has the ring around the ignition then locking and unlocking should not do anything.
If not then try it and make sure that you are getting an indicator for the car being unlocked. Hope that you do not need a new DAS module.