I think michaelgreen is absolutely right.
As far as i know there is no aftermarket sub that will fit to the housing without modification.
The sub also needs to be free-air version and needs to have a high efficiency because of the relatively weak infinity amp.
I also would recommend to use a stock chassis for replacement.
Try to get one from a wrecked car. That's cheaper.

The replacement of the subwoofers is descripted in the Crossfire service manual.
https://www.crossfireforum.org/forum...ollection.html
http://www.majormotorshotrods.com/_d...ice_Manual.pdf