I was very unimpressed with the Shaker 500 system in my 2011 GT, but wanted to keep the head unit for a few reasons (sync, a/c controls, stealth look, cost, etc).
The first thing I did was upgrade the door and rear deck 6x8's to Polk db 6x8's. This made a huge difference in the quality of the sound, introducing highs and overall quality that wasn't there with the factory speakers. I laughed out loud when I removed the factory speakers - they were paper and weighed about 4 ounces each - threw them straight in the trash. This upgrade alone totally changed the system, but there was still no low end.
So next I started researching the addition of a dedicated sub/amp. Luckily I pulled the sub and amp out of my old Mustang before I sold it, so I already had those items on hand. I ordered a 4 ga wiring kit and a Scosche line output converter from Crutchfield and installed everything, tapping the LOC into the rear speaker wires. I ran the power wire through the fender well on the passenger side as has been posted before.
I already knew what this sub/amp sounded like when hooked to the high end Alpine I had in my old car. After installing in my 2011, I was not happy with the response from the sub at all. I found out that the factory head unit turns the bass down as you turn the volume up to protect the cheap factory speakers. This was unsat. It was boomy at low volume and sounded like garbage.
Some more research led me to the MTX reQ. This device works as a line output converter, but also adds bass as the volume increases, overriding the factory EQ settings. I bought the reQ from sonicelectronix.com (used the make an offer button and got it for $69 instead of the $129 crutchfield wanted). Once I installed the reQ, the difference was amazing. The bass had a lot more power and was much tighter, it sounded just like it used to on my old Alpine head unit with dedicated 5v sub pre-amp outputs.
Anyway, here are some details on how I tackled some of the mounting problems:
The box I already had (Bass Slammer sealed 10" with Infinity sub) seems like it was made for my car. It is the perfect height to wedge into the spot at the front of the trunk where the floor is raised. It fits in tightly, and because it sits only on the raised portion of the floor, I can still lift the trunk floor without any obstruction to access the spare. It also still allows use of the full opening on the other half of the fold down seat. As much as I love my music, I didn't want to give up most of the functionality of my trunk, and this box wound up perfect. For reference, the dimensions of this box are 15-1/8"W x 12-3/16"H x 12-5/16"D.
To secure it, I screwed 2 eyebolts into the top of the box and ran some zip ties through the bar at the top of the trunk. This box is very secure and won't be going anywhere. I saw a post somewhere where a guy had c-clamps and wood pieces rigged up, which I was going to do initially, but I thought this was much simpler and just as effective. The zip ties are rated to hold 75 lbs, and I used two layers just to be safe.
Here is a picture of the reQ mounted on the back of the box. I picked this location so it was out of the way, and I can make adjustments to the settings by lowering the rear seat.
Since I was using a Rockford Fosgate Punch mono amp, I used the RF Punch EQ knob for easy bass adjustments. I found what I think is the best place for a bass knob, on the side of the console here. It does not interfere with the e-brake operation, and is in a very natural position for adjustment while driving.
I took 12v power for the amp turn on from the fuse box using one of these add a circuit things. Fuse # 35 provides power only when the key is on, which is what I want so the amp turns off when the car is off. It is listed in the manual as "auxillary body module". I have no idea what that means, but it works. You can also use fuse #29, labeled for the backup camera, but I was already using this slot to power my Garmin GPS. (On a side note to prevent somebody from an expensive mistake, if you want to hardwire your GPS, do not tap directly into 12V. It will fry your GPS because they run on 5V! What I do is wire a cigarette lighter to the fusebox and plug the Garmin power cord into that - the power cord has a voltage regulator that reduces the power to 5V.) The only drawback to the add a circuit is it's kind of bulky and I couldn't put the fuse box cover back on, but that doesn't concern me for now.
I ran the 12v turn on line into the console so I could add this inline cutoff switch. This allows me to disable the amp for whatever reason (getting the car serviced, etc). The little plastic panel the switch is mounted in just pops out, it is a great place if you ever need to add a switch for anything.
I decided that the 8" subs in the door might not be necessary, so I disconnected them from the factory amps on the kick panel. But after listening, I decided that it sounds better with them running, they definitely add some decent mid-bass. Surprisingly, I think that these speakers are decent enough and I don't know if an upgrade would be worth the cost.
I am not any kind of audiophile, but I am very happy with the sound from my system now. The highs are there, it is very clear, and there is enough punch from the sub to vibrate your nose hairs. I have no desire to upgrade the head unit or anything else, it sounds that good. I hope these ideas help someone else looking to upgrade the Shaker system.