I was thinking more along the lines of something temporary, like a hunk of acoustical tile laid over the floor boards in front of the back seat.... where he would get the most noise from and stares at. Not a complete fix, but something that could dampen things down a bit.
Another option... more of a training/conditioning option... would be to take a sound recorder and point it at the floorboards and drive around. Get some regular noise and also rumble strip noise. Put that in the computer or other playback device and play it at strategic times softly, then louder. Like you'd do to desensitize a dog to thunder.
If you do that at meal or snack times... when you are not flying down the road, trying to deal with a panicky dog... you'd probably see an improvement.