See above for question. I am going to use a single Pioneer TS-W304C driver, and am planing on building a ported box. I would like to know how to determine the exact sound characteristics of my car and then tailor a box to those properties. I have access to a computer with sound measuring software, and also have box design software.

Also, has anyone ever passive a passive radiator-style sub box for a third gen? If I can find an appropriate radiator, I may make one of them instead

Just given the ol' bump ^

You really can't get that precise just out of a box. The f-body has a transfer function that centers around 50hz, give or take. You could build a box that had a roll-off that started above 50hz, but your low bass output below the low end of the transfer function would suffer horribly. To get a truely smooth response, the most accurate route is to build the box to have as flat of a response possible on its own merit... not in a specific environment, then use a narrow band equalizer or a parametric EQ to flatten the in-car response.