@Pryme, Give Crutchfield a call. I got a lot of great tech info from "Hunter", his direct line is : 434.218.6124.
I think you have a WT Chev., with base radio, I have no exp. with. But "T" plug in harness adaptors are available for most vehicles, to not have to cut into a head unit wire harness, to add amps etc.. I saw one listing from the company NAV-ZEN. There are a few others, T-Harness.com, and LLJ Custom Stereo. Pre-made harnesses, and even made to order custom.
Most improvement in my truck was the "A" Pillar tweeters and Dash Corners speakers, that added clarity and stereo separation. Front door 6x9's do have a full range signal, so a Co-Ax would brighten it up as well.
My Denali also has tweeters in the top of the rear doors. I replaced those Bose with Kickers, and now I can hear them.