I'm pretty sure you're going to have to use the adapter to run the factory amp off your deck.

I'm planning on ripping my factory amp out and running my front speakers (soon to be components) off my SoundStream Reference 500s and my subs off my SoundStream Reference 1000s. (1000 watts RMS, class A amp).

Rear speakers will be powered off my Clarion deck or something since its just for rear fill.

 

You would use the Soundgate adaptor to hook the aftermarket deck to the factory amplifier. Replacing/rewiring the factory amp would be a waste of time. If you are going to do a 4/5/6 channel amp to run all of the interior speakers just wait until you install that then run new wires from the aftermarket amp to the speakers.

 

a few things work
you need some type of normal mitsu harness(every company makes one)
a) bypass factory amp(lose sub, speakers sound good on quality radio, use caps for tweeters or crossovers)
b) use soundgate-style adapter(hooks speaker wire up to factory amp, gains too quick, no fine adjustment, overdrives factory amp, distorts and can break over time, but easiest)
c) cut open cable(or use soundgate) get a 1 ft or other small rca, cut off ends with 6" of length, tap into wiring(either cut or through soundgate) and plug into rcas of radio(most difficult, best sounding, most reliable, gains correctly)

 

Choice a is the best. Run wires down to the output side of the factory amp and run them up to the aftermarket radio. That's a basic rewire folks, and takes about 45 mins to do. The problem on choice b, i see everyone is doing, you'll run into the same problem as in the eclipses with the infinity sound system. The amp burns out over time, you could get alternator whine, and will distort at higher volume.

 

^^^ the factory amp is really prone to burning out?

Also, i don't notice much in terms of distortion. But i'm also using factory crap speakers. I'd bet if I ran the boston acoustics pro series components which i'm buying soon off the factory amp i'd definitely hear distortion. I guess thats why i'm going to tear it all out soon.