The locking system on my 1992 500SL just stopped working. The lights on the door and trunk still work, however the only way to lock and unlock the doors is with the key. The key will only open the one door, not both. The internal locks are not working either. When I use the remote, the red light stays on for quite some time before going out and I hear a buzzing noise coming from the battery compartment in the trunk.... any ideas???