Okay folks here's my DIY to install an amp + subs. This DIY is only for those with enhanced premium sound (EPS).
Prerequisites and Materials
A) Determine if your car has EPS by running your vin at bmwvin.com. Enhanced Premium Sound is S752A (Individual audio system).
B) Order your Technic harness and LC2i from Technic. $145 shipped I think.
C) Purchase 2 amp kits, 1 for LC2i and 1 for aftermarket amp. These should include power, ground, remote, rca, speaker cables, etc. Spent about $50ish
D) Buy aftermarket amp and subs. Cartoys has a special right now which I didn't buy but it's a good deal. Two 10's kenwood subs, box, and kenwood amp for $299.
1. Once you get the technic harness you need to put it together. For some reason it was confusing as hell so I labeled all components and sent it to Technic for a dummies explanation. It was worth it because here's the key he gave me.
* B1 -> E1
* B2 -> A3
* A1 -> A2 (if adding aftermarket sub only) / C2 -> A1 (if underseat woofers are replaced with aftermarket sub)
* D1 -> E2
2. Provide remote turn on to LC2i. To do this, remove the rear passenger seat bolster (Yank really hard from bottom, wiggle a little, and pull up and out). Once bolster is off, locate the remote wire to tap. There are 2 black wires. You want to tap into the smaller black wire in the picture. Once tapped, run the wire to the trunk area to use later. I used the following link to figure out which wire to tap.
3. Remove trunk sidings and center cover. Google search it how to do this. It's easy. I removed everything to make it easier to wire it all up.
4. Unplug OEM harnesses from OEM amp. There are 1 or 2 other smaller harnesses. Unplug the smaller harnesses and get them out of the way. Now unplug the big harness by sliding the harness to the right. Btw, OEM amp is located on the left side of trunk.
5. Plug OEM harness into Technic harness and then Technic harness into OEM amp.
6. Provide LC2i 12v power, ground, and remote in. Remote in is from step 2 above.
7. Wire the remote out from LC2i to aftermarket amp remote.
8. Run RCA from LC2i to aftermarket amp
9. At this point you just have to provide power and ground to the aftermarket amp. Run speaker wires from amp to subs.
10. I mounted the amp to right behind the sub box. I mounted the LC2i right next to the OEM amp. Below is the end result. I'll take more pictures soon.