It is actually 3 things. The sub, the amp and the box. The sub is actually the least of the issue imo but it still is not a keeper if you want nice bass. The box is probably the biggest offender but the amp is a close second. The box is thin plastic and flexes so there is no low output just because of that alone. you can add deadedning to the box to help give it a bit of mass and lower the resonant frequency a tad but the volumn will not increase. You just get a tad lower bass note. This still results mostly in a dunk dunk sound but now has dunkuummm lol. Then there is the lack of any real power going to the sub so it gets drowned out by road noise very easily and this is even with our very quiet from the factory cars.

I would imagine that if you put the sub in a solid wood or fiberglass box and gave it 75w-100w it would sound decent and play lower. But then at that point you might as well get a better sub.

The factory sub is 3 3/8" or so iirc and the box is around .55cft. You can adapt a 10 into the box but the box is still plastic and will flex.

Now don't get me wrong the factory system is very clean for a factory system but it lacks any real output. If you are happy with the max volumn of the factory fronts then perhaps the factory sub will work for you but if you want anything louder then the factory sub will just not cut it.

For me when I bought the car I was impressed with the clarity but felt it lacking in bass overall. I played with the bass setting and never could get anything I liked out of the system by increasing the bass level. I disconnected the rears first since the right rear kept pulling the stage to it. This allowed for a little bit of bass increase from the HU but still it was just the dunk dunk with little volumn and it was still drowned out with road noise. So I added deadening and poly-fill to the box and it had a bit more bottom to it but still not like a real sub setup. While I had the sub out I had played with the bass level and found that the fronts could still play cleanly at higher bass level settings and did so with a deeper sound then the sub. After putting the sub back in I still was not happy even though it was a little deeper so I unplugged it and just ran the fronts as they were able to produce more bass output then the sub. Also since it was in front of me it did not get drowned out so easily.

Thanks for the replies i guess i cant just plug in a stock subwoofer

I am fine with the stock setup we have in our LT. I know its not as tight a bass as I could get from an aftermarket system, buts it pretty good.
I would recommend going to an auto wrecker and getting a stock sub enclosure. It has the speaker inside and the amp mounted on the outside. Then you can decide whether to upgrade the box, amp, sub or all of the three.
As for whether it will just plug into the head unit. You might have an issue there if the base single disc unit does not come with the sub. Check a head unit from a prem system for where the conections are....

There inlies yet another huge issue. You are correct that he would also need to add the Pioneer upgrade radio. This means that he also would have to take a trip to the dealer to have the BCM recognize the new radio. That is yet another $50-$100 right there.

Just get an after market set up and be done with it.

wouldnt be able to set it up with an aftermarket head unit?

Then trade off the cost of the dealer for the cost of the adapter kit. The cost of the aftermarket HU will probably be more then the factory upgraded hu but keep in mind how many people have issues with their factory HU so how long will the used on that one buys to save a few bucks last?

When it is all said and done it will cost nearly as much to upgrade to the factory amped/sub system as it will to get a decent little aftermarket system.

True. It may be too much trouble.....
however, you can get an amp or crossover that will accept the line inputs for the rear speakers to create a sub channel.
I guess it depends on what route you want to take.
Me...I like the stock look of the radio....so I would try to keep it....unless its too costly etc....

I also do not think that the factory amps 3 connectors are going to be found in the non premium stereo cars. So unless they are there adding the factory amp will not be possible without a harness upgrade to go along with it.

Yes this is what I would recommend. It is easier to add aftermarket then it is to upgrade lol. Although you will realize a huge sonic improvement by going with an aftermarket HU as well. The factory (premium atleast) HU pulls bass as you go higher up the volumn. It also pulls bass as you fade to the rear if you try to use that to balance your bass to your fronts. Basically the factory HU will eq out the bass the louder you try to get.