It should be ~12dB/oct, but it will start much higher than 35hz. Sedans start around 60hz, regular cab trucks start closer to 75hz, and suvs start around 50-55hz normally. It would be pretty easy to figure out though, just take a sub/box, drive out to a field, set the sub/box on the ground and measure it's FR. Then stick it in the suv and measure again, subtract the two and you should have something reasonably close to the suv's transfer function.