Ok the easliest way to do with using the factory wiring is this.....

Mount your 6x8 in the rear deck(make sure you pull out the mach amp rack back there.) Mount the speakers to mental part of the rear deck.

Now with the mach amp rack pulled out you will have 2 rectangular 2 pin harness back there that hooked up to the tweeters. Ok.

Now from your 4ch. run new speaker wire to the new 6x8s in the rear deck. And then run another set of new speaker wire and connect it to the 2 rectangular 2 pin harnesses. So now at this point you will have 4 sets of wire running up to the rear deck..... 2 sets going to the new speakers and 2 sets going to those gray harnesses.

Now when you install the door speakers DO NOT use the gray harness that is connected to the factory 6x8 woofer. You are going to use the gray harness that is connected to the tweeter....So just tie up the the woofer harness some where so it doesnt get damage. Now hook up you speaker using that tweeter harness.

Ok now go behind your headunit and disconnect the speaker wires from the radio harness that connects into the car.(the bigger rectangular harness)

Now this may get confusing......on the harness that plugs into the factory plug you are going to have to "loop" the speaker wires...Do this:
white to green
white/blk to green/blk
gray to purple
gray/blk to purple/blk
(do not do this with the wires coming off the headunit harness.....or it will feed back into the headunit)

 

Thanks Redfire! I see what you're saying: Run new wire to the rear speakers (since they are right next to the amp), and use the signal wire from the headunit to the old amplifiers as speaker wire for the front speakers. Tie this in with the tweeter wiring that was used from the headunit to the tweeters, and voila! Factory speaker wiring!

I like this idea, but I am still concerned that all of these wires run down the left side of my car right next to my 1/0 power wire... Would it be terribly hard to run new wire to the front doors?

 

you do not need to be concerned running speaker wires next to power wires, just rca wires that have not yet been amplified. There is nothing wrong with this method although you will need to find the bypass harness that I point to in my cardomain page, this is the harness you will want to tie the front and rear speakers together.

 

The only thing is you are not going to use the mach amp wiring.....When you drop the mach amp rack down in the rear you will see 2 gray harness that plug into the tweeters on that amp rack that is the harness you will want to use....I mean you can use the signal wires that are there for the mach system its really up to you.....Then like I said up front use the tweeter harness to hook up the the front speakers, do not use the harness that is plugged up to the front woofer...... Like ttocs said there no problem with the speaker wire running along the power wire just run the RCAs on the otherside and you should be fine.....

 

Ok just to be clear, you suggest running the signal from my amp through the rear tweeter wires to the front tweeter wires, to the front speakers?

Thanks for the help!

 

Yes, just because those 2 harnesses are not in the big mach wiring harness back there....When you drop that mach system down those tweeter harnesses will stay, bc they are ran on the top of the rear deck panel....I just think that is easlier using those harnesses.

 

this is an easy and commonly used install trick as long as you understand it. Make sure to connect them in the dash and you are golden.

 

^ I tried to explain this way to my buddy over the phone and he got very confused when it came to behind the HU for some reason. So when I finally had a chance to take a look at it. He had the rear speakers amped fine, but where he went wrong was he had the fronts powered off the HU and the amp senting powered signal to the HU amp.

I got into the car and did this when I saw the wiring.

 

he did it exactly the way you explained it, or so he said........

 

Yeah... I did it like you explained it...Was said.. No you didnt.

Which made it even worst I drew 3 diagrams showing what to do....3 different times like a month or so before he did it...