JL Audio makes a Stealth box for our PT Cruisers check it out at
http://www.jlaudio.com/stealthbox/ptcruiser.html
Also I believe MTX also makes a stealthbox well they don't but there is an option to place a request for one at their website
www.mtx.com
The JL Audio stealthbox won't take up any trunk space but will end up costing you around $400-$500 because it is a hide away system. The subwoofer itself is a 10" W0 which is their low end model, the sub is rated at 125 watts continuous which is just enough to feel it your seat but not enough to rattle the rest of you. You can also go with their E-Series amplifier if you prefer the E stands for economy because JL Audio is quality stuff much like Alpine. Its not cheap but you get what your paying for. Great sound and trunk space without a clunky box taking up 3/4 of your trunk. You can put the Amp underneath the passenger seat or driver's or you can have it mounted to the back of one of the fold downs, the amp will not be very big, however you will need an amp install kit and if you take it to a shop(which I recomend, because if anything goes wrong its their fault and not yours) they will charge you an install fee on the kit and an install fee on the sub box. There is but only two other alternatives if you want some bass in your car. Get a single 10" sub in a sealed or ported box and place it in the trunk now you will lose some trunk space but not a great deal. And if you decide to go this route you'll have a much better choice in Subs from moderate to extreme. The same can be said about the amps they too will fluxuate. As does the price. So the other alternative I metioned is just upgrading the factory speakers. This too can get exspensive quickly. Not only do you have to pay for the speakers themselves you have to pay for the labor on each pair, but not per door. There is 6 1/2" components up front (6 1/2" speaker in the front 2 doors and 2 1" tweeters one in each corner of the dash) and your rear fill is 5x7". However if you take it to a shop or Best Buy and try and get your speakers replaced someone will mention to you trying to upgrade the factory headunit and add on an Amp to the Highs and Lows, so a 4-channel amp will be used to handle all the channels along with another amp kit and labor on all of that. So it can become quite overwhelmiong in a short amount of time. Most CD players now a days are MP3 compatiable and start out rather cheaply at $150 and up. But here again at a shop you'll pay for a dash kit and a harness and sometimes a labor fee on those plus the install of the deck if its not already included in the price. I use a Alpine headunit and a JL Audio sub. I've trusted and used these brands since 98 and it has never let me down as far as performance and sound enhancement. There great products to own. I hope that this helps you decide on what you want. You can get everything I've talked about today for around $1,000 to $1,500 I know thats alot of money but the stealthbox alone is 400-500 if you throw in a Amp for around 150-200 and the a amp-kit for 50-60 and then labor and tax and it should cost you around that price or close to it. Here again I hope this helps and let me know what you decide on doing.[?]