I'm almost certain that the stock radio isn't compatible with MP3s or WMAs.
As for your satellite radio, you've got a couple of options:
1) Universal kits that include an FM modulator. They're decent but you rely heavily upon the reception between the modulator and your stock antenna. If you're going for functionality and not sound quality specifically, this is the cheapest route to go.
2) A kit directly from Mopar or your local dealership SHOULD be able to be integrated into your factory headunit. It will be more expensive, especially if you go to a dealership. You may want to contact your dealership anyway, pitch the idea to them and see what you need. You don't necessarily need to order the parts through them, but they'll think you want to and will therefore be more than willing to help you out. :P It'll look something like this:
Mopar OEM Chrysler PT Cruiser Sirius Satellite Receiver Kit 03-Up - AutoTruckToys.com
3) Aftermarket headunit with MP3/WMA compatability that offers an EQ band that consists of more than just "Bass" and "Treble" and is also Satellite-Radio Ready. Most units you will find will work with same-brand headunits (i.e. a Pioneer XM kit will work with any Pioneer headunit) and in most cases will connect directly to your receiver. This will produce the highest sound quality and will probably cost slightly less than the satellite radio kit from the dealer or Mopar itself for your factory radio.
Hope this helps.
