The battery that operates the remote lock/unlock in one of the keys for my '97 E420 has apparently died. Is this something that needs to be replaced by the dealer, or can I do it myself (and if so, how do I reach the battery)? Reading some other posts about the "handshake" system used in these keys makes me a bit nervous about doing my own R&R, but I hate to go to the dealer for anything if I can avoid it! Thanks for any help!