A suggestion is to use RCA Y-adapters to get the audio from your existing setup to the new after-market amp. Plug the Y-adapters into the RCA plugs behind your HU and connect the existing setup, and the new setup. Run the new RCA cables to the new amp. Good Luck with your stereo.
2006 GT, Strut Tower Brace, Gas Hood Struts, FR Intake Plenum cover, Metal Fuse Box cover, Moroso brake reservoir cover, Rear shock tower brace, Front Bumper Lip Spoiler, JVC DVD Head unit w/ back-up camera, satellite Radio and USB input, 2 - 12" sub-woofers, 4 - 6"x8" speakers, 2 - 6.5" speakers to replace the door subs, 3 amplifiers, LED lights under the hood and in the trunk, chrome e-handle on parking brake handle, chrome cup holder trim, and trunk lid cover with running pony emblem.