nice job. I did something similar copule years back in college.
made out of junk from ace hardware. it is mounted to the seatbelt bolts.


I also saw this at the time but never used it. I imagine it might work for smaller cameras.

but now I have my pedco ultraclamp and triple suctioncup mount that clings to windows and such. works fro my digital 8 you see in the pic above. not sure it would work for larger cameras.
here is a pic before I spun out and wrecked my car on the track.
you can see my gf's mini dv sony camera mounted to the bumper. unfortunately it wasnt recording AAAGGGHHHH haha.

oh yeah and the silliest thing I saw but works pretty good is a simple tripod. I saw it on another track. a guy with a camaro, I think. Anyway I did it. I extended the legs to the floor and the third one out to the back seat. I then used bungie cords and attached it to the center hook under the center post of the tripod. I anchored it down to the floor using the front seat rails and brackets to latch onto. unless you are pulling some seriously hard turns, the tripod will stay put very well. again with larger cameras im not so sure how well this will work for you.
Another thing you can do, which requires more work but a much more stable shot, is to have a "c" pillar bar. The "C" pillar is the pillar behind the rear passengers. the bar bolts to the passenger seatbelt mounting points and adds increased rigidiy for handling. here is an example in a 92-95 Civic hatchback.

the ultra clamp can mount to bars. so you can imagine a simple clamp mounting to that fixed bar.