Try going to the part department at a Mercedes dealers and asking nicely for them to help you. The direction in your owers manaul should work. Two year ago I had the same problem you're having with a key I put a new battery in. I couldn't get it to resync for the life of me and ended up buying a new key from the dealership. I think it was about $125. Good luck.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230