Hi - I would always recommend a diagnostic check on a tool such as Mercedes STAR diagnosis, this will often have fault codes stored relating to issues like this and may help inpoint the problem.
A very common problem however is a faulty stop lamp switch (even though the stop lamps may work just fine)
This is very cheap & easy to replace - it is located just above the brake pedal
If you did not want to have a diagnostic check carried out then I would replace this first
Another problem can be the selector module itself but I would recommend strongly having a diagnostic check carried out before any other repairs are carried out if it is not just the stop lamp switch!
Best regards, Paul
Obviously Merc diagnostics is an option but I would have thought that there may have been a more common fault after a battery is drained and then charged.
Hi - I have given you the most common problem! A faulty stop lamp switch, as said I would try this first if you do not want to go to Mercedes for a diagnostic check.
Unfortunately with modern computerised systems you do need diagnostic equipment to correctly diagnose many issues, I am sorry that you think my service is poor but it would appear that you do not really want to hear what I need to say!