I'm not sure about the wiring setup but i am willing to bet that its different, the mach 460 has seperate amps in the rear and a component set up front. Isn't your just a 4 speaker setup now? I think the mach 460 is setup for 6 or 8 (not sure about how many in the rear).
Might get more replys if you post it in the Sound and Shine forum as well ?
If you don't have the amps, pm me because I have the ones that came out of my GT and they're just sitting here. But I do believe you will want to have everything hooked up to the unit, because it was designed for a total Mach 460 system. I know when I wanted to hook my subwoofers up, I couldn't use the stock headunit or the amps because of the wiring distortion it made when hooked with the sub amp. I had to buy an aftermarket unit, wire my mach 460 to the subwoofer amp, and tune the tweeters to work perfectly. Take a look at the factory amps. If you have them don't worry. Oh I put a pic of my subs in and how I built my box. It's awesome. I put the new RF Punch Stage 3 10" subs in with a 659 watt RF amp. It hits very hard, makes my radar detector dance and makes a pain in my ears. AWESOME.....
If you have a 6 disc CD in your car and the 2 tweeters on the door panel, then you most likely have the 460. It's a good stereo, good sound and stronger speakers as compared to other factories, but it's lacking bass badly and it makes customizing your stereo system a PITA. Their wiring is obsolete and complicated, and makes you dissapointed. If you have the Mach 460, the rear speaker screens will say so.