The trunk lock on my '87 560SEL seems a little broken. I can unlock it with the key, and doing so will unlock all doors, but I cannot lock the trunk without locking one of the doors (which locks all doors, including the trunk). Obviously this isn't a big deal, but it can be annoying. If I try to lock the trunk from it's lock, the key will barely move, I assume it should go one quarter turn clockwise. Any ideas?