I made a classic rookie mistake and installed some Kicker CS68 6x8's on the factory base head in my '13 nearly a year ago. Needless to say, they aren't as efficient as the stock ones, so I found myself having to turn it up quite a bit to get decent volumes. That of course led to clipping which ultimately killed the tweeters on the front speakers. I knew better, but I did it anyway. It did sound a good bit better while they lasted.
Fortunately, I had Geek Squad Protection on them, so I was able to get them replaced for free, but I did not want to keep on doing the same thing. I also did not want to run a bunch of wires to an amp, so I stumbled upon the Kenwood Power Pack: Kenwood KAC-M1804 (kacm1804) 400W 4 Channel Class D Compact
It is 400w Max and 180w RMS. That's a perfect range for the CS68s that I have. It takes speaker level inputs and amazingly it fits in an empty space just below the head inside the dash. It took me all of 2 hours to install (though I have installed several systems over the years). For anyone that needs it, the speaker and power wire colors can be found here: BaseStereoWiring.jpg Photo by thevikingnj | Photobucket
It just about doubled my volume (or more importantly, allows me to get good listening volume at nearly half the level). It is a Class D amp, so the sound is a little crisp and unnatural, but that class of amp is the most efficient. This is important when you're going to put one in a tight space due to lower heat production.
It's by no means an audiophile setup, but with this power pack, the CS68's and a Kicker Hideaway Sub under the rear deck, I have a 700w Max/ 330w RMS system for about $500 and a completely stock interior/trunk. I would say the sound is at least as good as the Shaker 500 system for less money.
Anyway, I thought I would share my experience. The big news is that you can comfortably fit this or a similar amp inside the dash. It is designed for this purpose, so I hold out high hope that it will not overheat in the Texas summer heat. Only time will tell...