ctuna's isntructions worked! See post #14 on this thread. Thanks for the solution! And credit to my wife for finding this post.
To build on it, I figured out how to do it without having to strip a wire, or anything else invasive. This should also save you the uncertainty of hunting down the infamous grey wire with green stripe amidst bundles of wires. I'll post a video later. Note: I was working on a 2011 328i with a disconnected battery and a locked trunk.
If you're a layman like me, don't be intimidated by any of this. It's very doable. I'm not a mechanic or an electrician, so please pardon what is likely my ignorance of many things. I just want to be more helpful than "put your key in the keyhole and turn".
The goal is to be able to pull out the junction box ("A4010") to disconnect and access the backside connector ("Connector X14270"). Everything on a BMW seems to work like a Japanese puzzle box, so you need to do these things in order and reverse it when you put it back together.
You'll need a torque screwdriver, a phillips-head screwdriver, 1-2 feet of radio wire, and a 9-volt battery. Electrical tape, 1 small wire clamp, and a plastic prying tool are handy too.
1. Per ctuna's reply on this string (#14, line 4a), remove the bottom panel below the glove box.
2. Remove both cup-holders above the glove box. This is necessary for the glovebox to pull out (next step). You'll need to pop out the plastic trim around the cup-holders. Use a plastic prying tool if you have one, otherwise a flathead screwdriver will work - just be very careful not to scratch the trim or your dash. It should give easily; just pop one side and pull the rest out gently with your fingers. There should be 4 screws. After taking them out, the cup-holders should pull right out.
3. Remove the glove box. I think the reason ctuna couldn't manage to pull the junction box is because the glovebox locks it in. Open the glove box and there should be 6 screws. Once they're removed, the box should pull right out. Disconnect the connectors of whatever electronics are hardwired into it. I just had a USB charger and the glovebox light. Now you should have full view of your fusebox and the flat junction box below it.
4. Disconnect the junction box. First, disconnect the two wire connectors facing you (one blue and one black) and secure them out of your way. Next, there are 2 screws on the left and the right of the plastic "sleeve" holding the junction box secure. After you unscrew them, there's a plastic tab on the top of the box near the left screw. You'll have to feel around and press it down to unsecure the box and be able to pull it out. Note: You shouldn't be able to pull it all the way out - nor should you just yet.
5. Once you have the junction box loose, disconnect the wire connector behind it. This is the trickiest part. On the far right (as you face it from the seat) on the back of the box is a tab. Slide it all the way out to the right. It might take firm force at first. That should cause the connector to pull away from the box. And there you have it! That connector is the Grail. Pull out the junction box completely and set it aside.
6. Locate the correct pin hole (pin #12) on the connector for the trunk release. You'll notice the junction box has corresponding pins to send electric signals down these small square holes. On the connector, you'll notice the top row of holes has a "1" on the far right, and a "20" on the far left. Just count right-to-left to the 12th hole in the top row. On the other side, you should see the infamous
grey wire with a green stripe. It's almost dead-middle of the connector.
7. Send a small electric current down this hole. I used a 9-volt battery and thin radio wire (usu. 2 very thin intertwined wires with a red and a white insulation for positive and negative respectively). To ground it, I wired the negative lead of the battery to a small clamp (with my white wire) and attached the clamp to the passenger seat rail. Now, I was paranoid here not to let your positive and negative wires touch each other or the same piece of metal for fear of shorting or burning something out. It's just a 9-volt battery, but better safe than sorry. Next, before attaching the other (red) wire to anything, I stripped about an inch of insulation off one end and shaved the wire thinner with a knife. You should then be able to slide that end of the (red) wire into pinhole 12. Be careful and don't force it; you don't want to end up with a broken piece of wire jammed into that hole. Once that end of the (red) wire is secured into pinhole 12, just brush the other end of it against the positive lead of the battery.
If all goes well, you should hear the beautiful sound of your trunk latch releasing and a weight of embarrassment (for getting yourself into this mess) lift from your shoulders.
PS - Just in case, fold down your back seat when you're done.