Obviously, the first way to open the trunk is to use the trunk release button under the dash. Next, the key in the trunk keyhole. Finally, the only way to climb through the back seats is if they have already been released from the trunk (a catch 22). Last resort is a locksmith (or having BMW or another certified BMW mechanic check it out). This actually happened to me before and the key wouldn't work since it was too cold. Luckily, one of the back seats was unlocked and was able to pull it down and climb in the trunk to release the mechanism manually with a screwdriver. It worked after that. Must've gotten jammed. Hope this helps.