Audio Systems: 2004 sebring firewall etc..., wiring harnesses, amp power

Question
Hi, i just bought a new car a 2004 sebring and i want to put my old audio system in, i had a 1994 Explorer and it was my first time putting a "system" in and all i did was went to canadian tire looked at a wiring kit and the instructions on the back went home used my own wires and got lucky i guess what i did was used the proper power wire (a friend gave me) from the battery to my amp, a wire i took from an extension cord from the blue and white wire on my deck to my amp and i grounded it out with a speaker wire to a bolt that was back there bolted to the seat or something lol. so first off is this proper? if not any suggestions? also i heard the sound quality is really noticeable when you buy those expensive wiring kits is this true? is a 600 watt panasonic amp powerful enough to hold 2 10" subs in the same box (not sure of the ampage) CVR's anyways. and the main question i dont have direct access to the battery in my vehicle its just a booster post and the harness or whatever that holds all the red wires is attached to it i hope you know what i mean but anyways i took a quick look and there is no hole in the firewall on the drivers side (thats where the battery is) did i not look enough or do i have to drill one myself? because my explorer already had one on each side with a rubber cap. i know i can just go get this done for me but i like to learn and do things on my own so if you can help me out that'd be great and i appreciate you taking your time to help me thanks.

Answer
I hope you have a package of hot dogs handy as that will be one nice fire.

You must use the correct gauge of wire for the amp - power and ground must be the same size wire on an amp.

You must also have a fuse of the proper rating no more than 18" from the battery (+) to the amp.

You can drill a hole but you must know what's on the other side AND you must use a grommet to prevent the power wire from shorting out on the sharp metal.

You can get all the right wiring harnesses and install kits from here http://www.metraonline.com

Here is a guide that will calculate what size alternator and power wires you need and how to set the gain correctly on your amp  http://www.datafilehost.com/download.php?file=6d26c621

You will need Microsoft Excel. If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g