Simple stereo upgrades?

[bgonline]

I'd like to add a subwoofer to my 325xi wagon. It has the stock Alpine head unit, no CD changer or nav, no XM. (I did add an Ipod jack)

Is there an Alpine subwoofer/amp I could buy that 'plugs in' to the BMW wiring harness where the CD changer compartment is?

Anyone made any 'minor' upgrades to improve the sound?

Thanks in advance-