Adding a subwoofer

[PW]

I would like to add a subwoofer to this radio
http://www.aatechnologies.com/cmyradios/chevytrucks/ulod.asp
that also has this cd player
http://www.aatechnologies.com/cmyradios/chevytrucks/rmtcd.asp
and I was wondering what the best way to do this would be.
Is there a wiring harness I can buy that will allow me to add a
powered subwoofer and keep my current front and rear speakers.
Thanks in advance, Phil Woolfe