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.
Hmmm why didn't I think of that? I'm gonna go and try it. Thanks! I'll post back to let you know if it works.