The ski-pass can be used to transport long objects, but the opening can also help audio nuts like myself to allow more of the bass from the trunk sub to penetrate into the cabin.

If you don't have the ski-pass in your car, you also don't have the fold-down seats, which is hard to understand why BMW did it this way. So this DIY is for those who have the non-folding rear seats.

Keep in mind that once you cut, there is no going back. You will have a hole in the back seat wall. You can hide the hole from the cabin side, by flipping up the armrest, and/or by reinstalling the OEM plastic cover, but from the trunk side it will always be visible, unless you re-carpet the back seat wall. The edges of the hole can be "prettied up" with some aftermarket trim, but what I am planning to do when I sell the car is install a carpeted cover plate on top of the hole.

So, let's get on with the fun!

Tools:

• Power drill
• large ruler or level
• Sharpie marker
• Carpet knife
• Dremel with multipurpose cutting bit, or
• Jigsaw
• Vacuum cleaner
• Safety goggles

Step 1: Removing armrest plastic cover

Flip down the armrest. You will see a black plastic cover.



You see that I drilled a couple of holes in it because I had trouble pulling it out from the sides. In my case I am installing a subwoofer in the trunk so I will plan to re-install the cover but I will cut out a large section in the middle to fit a grille, so the hole did not matter. If you plan on keeping the cover in one piece, you will need to pry from the sides. The cover is held by two plastic "dowel clips" (probably not the right word for it) and you need to pull pretty hard to release the cover.

A picture of the back of the cover:



Now you can see the plastic trunk wall:




Step 2: Drilling the corner holes of the opening

What you want to do is drill holes from the inside of the cabin through the trunk wall at the corners of where the opening will be. In my case, I wanted to maximize the size of the opening, so I drilled in the following spots (marked with black circles):



If you can live with a smaller hole, and/or you want to maintain the two holes for the plastic clips of the cover so you can reinstall it later, you will want to drill the holes below that area.


Step 3: Marking the cutting lines

Now that the corner holes of the opening are drilled, go into the trunk and connect the dots, so to speak. I used a common level but a long ruler will also do. Make sure though that the cut lines are level, since you don't want a crooked opening (that's why the level is better). You want the opening nice and square. It'll look something like this:



Next, you want to trace the cut lines with the carpet knife, making sure to cut all the way through the carpet. If you don't do this, you may get some frayed carpet edges from the cutting tool. If you use the Dremel to cut, you may also end up with carpet fibers wrapped around the bit, not cool!




Step 4: Cut away!

Using Jigsaw: You will need to drill extra holes for the vertical cuts since there isn't enough room to place the jigsaw at the corner holes. You will want to start in the middle:



The horizontal cuts can be started at the corners:



Note: Depending on how close the top of the opening is to the metal trunk brace, you may not be able to use the jigsaw for the top cut, since the jigsaw's cutting plate (?) may be in the way. In that case you need to use another cutting tool, like a Dremel.

Using Dremel: It is more difficult to cut a straight line with the Dremel, so go slow and take your time. The Dremel always wants to pull in another direction, so you need to hold it firmly with two hands.

Once you're done cutting, it should look something like this:



Congratulations, you can now go skiing or enjoy better bass from your subwoofer.

__________________
Mobridge DA2 > JBL MS-8 > JL Audio HD600/4 > (3) Hybrid Audio L3SE & (2) Rainbow Cal25; JL Audio HD900/5 > (2) Morel HCW-10 & (1) AE SBP15