Mine is mounted on an L bracket on the outside of the spare tire compartment (74 VW bug). I figure anywhere you can mount it to metal in the front left quadrant where it is easily accessible is good, with the idea that anything that can hurt the driver should trigger it. I don't remember if it's necessary to be vertical or not, mine nearly is.
You can just see the top of it in the attached pic.
You can just see the top of it in the attached pic.