Just made a nifty camera mount for my 03 M5 that cost about 1/4 of anything I saw on the web.
Get a cheapo tripod with a standard camera/video mount at any camera shop. Just make sure the piece that the mount connects to and moves up and down in the tripod is at least 16-18" long. All mounts will let you adjust pan/tilt/level.
Remove this piece from the tripod and take it to your car. Lay it in the desired position between the seat and headrest and mark where the 2 headrest bars meet the piece.
Go to Home Depot and buy a piece of metal (could also be wood), and a couple of fasteners which you can tighten down (basically just a couple of bolts with a wingnut fastener, or a nut and bolt with a knob, etc. Just look around for whatever looks good.
Drill holes so that the bolts can go though the metal piece and into the tripod piece on either side of the headrest poles. Attach to the headrest, add your camera, and roll tape!
The only thing you need to think about is which type of fastener to use. If you are really lazy you could just get the right lenght bolts (2) and use standard nuts. Only thing is you'll need a wrench or ratchet to install and remove.
I also glued some rubber on the inward facing surfaces for a non-slip grip. I was also able to find bolts that had a black plastic knob on the end, so I can install and remove without any tools.
Enjoy.