allright i'm sure i am freakn retarded

but i want to add a sub or two to my contour. Currently i have 4 pioneer 6x9's installed in the doors already and a panasonic mp3 cdplayer headunit that has like 40-50 wats x4 channels or something like that (can't remember don't have the book)
my question is... i want to add a sub or two and i'm not sure how to go about doing it. do i just add a sub and amp to the sub and feed that to the headunit or do i fead the door speakers to the amp as well as the sub then back to the headunit or what?
i'm not quite sure what you guys normaly do but i want it to sound nice,
anyboyd have any suggestions on subs, amps, and setup?
i looked at that basslink (yes i did go back several pages looking before i posted) and its out of my price range
any help is appreciated thanks