Tricks to Unlock the Trunk With a Mercedes Key Fob A mercedes key fob lets you unlock your doors and start your Car Key Replacement Specialist directly from the driver's seat. It also has a panic key that locks all doors and activates the alarm system. If you are having trouble using your Mercedes key fob, or notice that the battery is near death's door, it may be time to replace it. Here are some tips to make it safe and effective. How to Unlock the Doors If the mercedes spare key key fob doesn't work to unlock the doors of your vehicle, there are a few options you can try to get it working again. First, ensure that the key fob's batteries are still in good condition. This can be accomplished by checking the small RED light that is located on the end of the key to confirm it's functioning properly. If the RED light doesn't turn on, it could be a key fob that is defective. If it is, you need to replace it.