Sometimes I wonder why BMW does certain things. In this instance, why did they choose to remove the key lock mechanism in the trunk of cars starting in MY2009? I recently had to change the battery on my E82 MY2011 BMW 128i, in which I removed the old battery from the car so I can use it as a core. Then closed the trunk and locked the car by using the key in the driver's side door.
It's at this moment I realized my day wrecking mistake. (Facepalm) Yes, I can get in the car, but the button to open the trunk requires electricity! On the E82's, the seat release is also in the trunk, so you can't even put the seats down. I was like, "be calm, just hook a battery charger to the terminal under the hood and we will be back in business." Nope, it didn't work. Come to find out; this method will not work unless the battery is still in the car and the positive connector is still attached to the battery. I even tried hooking the car up to my Jeep with jumper cables in case my battery charger wasn't providing power since it didn't sense a battery.
I then found out I can go in the car's back seat and remove the passenger side rear shelf speaker to access the passenger side seat release. I was skeptical and but it worked. It only takes 5 min max, and the majority of that time is getting the correct socket. One thing to note is when you are pulling the release cable, pull it hard. You might think you're going to break something, but pull hard and towards the front of the car. It will be okay.
I hope this helps someone, and I made a quick video to illustrate the method as I tried and released mines. It should work for all 1-series BMW cars. For the 3-series, that's a whole other method.