[28-80Hz Issue]

I think you're sitting in the middle of a bunch of resonance and cancellations coming off every part of the inside of your car. With the given information, I think you need to deaden everything, but it may not really work because windows can't be damped and the dimensions of the car cannot be changed. 

[Center Slot ported box for 2 Rockford Fosgate P3D4's, tuned for everything from rap to country...35hz???]

Joe X design is a good ported box. Corner 45s don't do much for a subwoofer box besides brace it and eat up internal volume. If you wanted a similar design with a bit more internal volume then I do have one free on youtube. Feel free to do basically whatever with it.

 

 

You could also try to design a mass loaded transmission line with offset driver/stub. It's basically a lot like a ported box but attempts to cancel resonance in the enclosure to give more bandwidth.