the 12 volt thing may not work due to the fact that in a mercedes the trunk pop button probably just triggers a relay. But under the hood there should be a place to jump start the car with jumper cables. If you can find this hook another car up to it with jumper cables like you would jump the car off. this should power anything in the car so you can pop the trunk. even thought the battery is in the trunk most of the cars with a trunk battery can still be jumped from under the hood.