07 jeep commander stereo

A friend of mine wants me install a system into his 07 Commander.  I used to install professionaly for about 5 years, but I left working at mobile electronics shops about 3 years ago to become a grease monkey at a dealership.  Basically Ive been out if the game for a while and Im not fimilar with some of the new vehicle wiring.  His Commander has the Boston Acoustics Premium sound currently.  I plan on building a fiberglass sub enclosure, depending on the amount of factory speakers in his jeep 1 or 2    4 channel amps, a sub amp, and a single din monitor/gps/stereo.  Does anyone have any pointers for bypassing the preimum sound?  Is there a harness made to bypass the premium sound? or do I need to rewire everything?  Are the factory amps built onto the speakers or are there seperate amps isolated somewhere else in the car.  What is the resistance of the factory speakers and can they handle aftermarket amps.  I havent even looked at his car yet, im just trying to prepair myself ahead of time.  Any help,tips,and pointers would be greatly appreciated.  Thanks