Check out JL Audio's Stealth Subwoofer for the PT Cruiser
http://www.jlaudio.com/.
It's a 10" unit that fits inside the rear panel just like the Mopar unit, but uses a separate amp. JL also makes replacement door and rear pillar speakers that work great with their subwoofer. Find a place to hide your amps under the seats and the whole installation will look stock, but sound much better!