The bose "sub" is most likely an odd impedence like 3 ohms. You won't find a speaker that will drop right in and work like the bose one. If you blew the speaker, it should still be covered under warranty (depending on when you bought your car I guess).
Many people just upgrade from the stock system so you might want to start a thread in the WTB section (its a sub-section of the For Sale section) stating that you want to buy the OEM 5.25" sub.
...or you could use this opportunity to upgrade to a real sub, amp, and enclosure.
