It might be worth talking to your Mercedes Dealer. A few years ago my brother had a Mercedes SL350 and lost the key. The SL350 used simple keys unlike the Mercedes of today. The local Mercedes Dealer was able to order one from the VIN.
Failing that if you are nearby to a locksmith they should be able to open it for you. They might then be able to cut a key or you will have to buy a new cap with key.