If you want to use the factory amp the audio inputs are not a problem, the problem is a turn on for the factory amp it is turned by the CAN bus signal FROM the factory radio.
You can get the metra Harness #70-6507 and it has enough wire to go to factory amp location (driver knee bolster) and a jumper for the ignition wire, the only useable wires at the radio opening are the ground and the constant 12volt, no illumination etc..
I ran my Kenwood to factory spkrs (boston acrapstics)for about a week and it works fine at low volume but the fac spkrs are low impedance spkrs and the radio would shut off due to that. you could get a small 4ch amp and just hook it up using the fac amp power wires with no prob. Blaupunkt used to make a nice small amp but I just checked and they don't seem to have them anymore except for this one PCA450http://www.blaupunktusa.com/Blaupun...s/ProComponent/
it is like 9x2x12 with a little fitting it could go where the fac amp was.
i am sure there are some other good quality amps that will not cost alot.
On the other hand if you just wanted to use the kenwood head units power just change the speakers to a nice component set up front and a pair of 6x9s in the rear. Sorry I am rambling....
I would get a very good quality 4channel amp and run it to your factory speakers you will need some kind of crossover network a good shop should be able to build you one. The Factory amp uses the Can-bus to turn on /off and it also has the crossovers built into it for the speakers. You can probably just put a "bass block" type on the front dash speakers, and then a bandpass Hi/lo unit for the doors and the rear spkrs I only use for a little fill in anyway I wouldn't sweat those too much. the factory speakers are low impedance but a good 4channel amp should run them with no problem most good amps have crossovers built in so you can use it I know it sounds complicated but it is really not. You will eventually replace the fac speakers anyway they are just cheap paper cone dodge speakers with Boston Acoustic rubber stamped on the magnet. You could get by with just changing the fronts. I would reccomend Focal, Diamond Audio.Kenwood has some really nice 7" that are pretty nice.
Stay away from best buy and circuit city and any shop that sells audiobahn and claims it as "high end"
full link http://www.lxforums.com/board/showthread.php?t=7902&page=1&pp=10