If all it is is just the foam surrounding the speaker, don't wasted your good money buying a NEW or even USED replacement sub woofer and amp, just do what I did. Fix whats broken.
There are 4-1/2" nuts holding it above the spare. Remove them, unplug the amp and take it out being careful not to damage the foam ring on top of the speaker. Take off all the other foam padding covering the speaker and remove the four screws holding the speaker.
Next go to Google and do a search for 'speaker repair kits'. I found Speakerworks and spent $13.62 including tax and shipping and the speaker works fine.
It took me about an hour to removing speaker, fixing crack and installing. Not counting letting it sit overnight for the glue to dry.