First of all if you are looking to upgrade your stock speakers at all I suggest you check these out...Sony xs-V1633....I have them and generally I don't like Sony audio products but these speakers are pretty nice and give a full range of sound. Right now they are only $15 for three on EBAY but the auction ends TODAY. I have my rear speakers disconnected and have an amp hooked up to these in the front doors (this is more than enough sound for the average listener).

Generally you get more quality sound from an aftermarket head unit . A lot of this is due to aftermarket head units having a built in amplifier. This allows you to turn up the music louder and have less distortion. RCA outputs in a aftermarket head unit will have less chance for distortion and be better sounding since you do not have to mess with speaker wire at all. The connection is directly to the head unit. Also aftermaket head units are usually better quality so the sound in general is much cleaner. The stock head unit to my knowledge does not "boost" any wattage because it does not have an amplifier.
My suggestion is that if you are just wanting to hook up some subs and don't usually crank your music up. Plus you don't care if the quality of the sound is perfect then you probably should just tap into the speaker wire behind the head unit. If you are going to try to keep improving your sound system then I think you should just go ahead and save yourself the trouble and just buy an aftermarket head unit. You don't even really need to spend that much money on one...I bought the cheapest Pioneer head unit (DEH-1700). If you get it in the store it's around $80 on ebay it appears to be around $55 or less.

If you want to create a really lound and at the same time clear sounding system you should think about buying another amp for your speakers as well as upgrading to aftermarket speakers. If you go to my registry you can see the system I have....it was not very expensive but sounds great. However, I would not suggest that you buy any sony car audio products. Ebay is a good source to upgrade car audio. This summer I spent $60 which includes the amp and wires (the wires cost me more than the amp) to power my speakers. Now my system sounds 20x better for not that much money. If you are really interested in setting up a better sound system let me know and I can tell you what I did and what you can do.
Another suggestion is to install your amp under either the passenger or driver's seat of your car b/c mine kept overheating in the trunk. However, with that said my amp does not have an internal fan.


Includes EBAY estimated costs of my sound system...just so you can decide:

$55 head unit...Pioneer DEH-1700

RIGHT NOW $15 set of 3 speakers....Sony xs-V1633

$70 equalier...Clarion EQS-744....I got it for $40...it's not necessary but if you have a cheaper head unit like mine then it allows for you to easily hook up three more amps...plus it allows you to fine tune your system. Another perk is that it allows for an auxillary input so you can hook up an ipod or any other type of cd player/mp3 player/laptop whatever to it.

$30-$40 amp....Lightning Audio Bolt B2.150.2....I was a tad bit skeptical when buying this amp but I'm satisfied after using it. Lightning Audio is by Fosgate so it is a fairly good quality amp. It also got fairly good reviews from what I have seen.

$30 for wires....not off ebay...it's better to buy wires by the foot if you already have your amp hooked up since you can use some the pre-existing wires

And if you put an amp to your speakers or have a large amp powering your subs...you should probably buy a capacitor if you don't already have one.