It is gonna sound like crap if it is even possible..a couple of problems.
1. You will have to run your rear speakers in mono
2. Finding speakers that can handle 600 watts
3. The power will be continuosly going to the rear speakers at 600 watts at the same time.
4. This amp is not designed for this, it is designed to run subs either in series (bridge the amp), or in stereo
You would have to run 2 to 1 rca's out of the rear of your head unit (if you have pre-amp outputs), or splice your hi/low wire connections. You have a bad ass amp (as far as power goes...don't know the brand)...get some good subs and have some fun. Then get a different amp for your cabin speakers.
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Schleprock, Schleprock...stronger than steel!