Looking for 6 inch subwoofer

Sorry if this question has been asked a million times  before but I lost 
track of car audio components a few years ago and it does defy googling 
somewhat.

I have a space for two 6 inch speakers in the rear doors of my car and
want 
to put a subwoofer into it. The front has space for a passable mid range 
unit of about the same size.

Has anyone developed anything with any sort of extended bass which can fit

into this space yet? I know it is TOO SMALL but was wondering if anyone
had 
developed any technology which could do this? Such as a long-throw cone??