I have 2, and did so for 2 reasons:
1 - my preferred mounting in the car wound up being roughly in the middle of the cockpit on the petty bar (diagonal from the main hoop to passenger floor downbar of front hoop) as I couldn't reach the area to the left of the wheel due to seating position. the wheel is extened a good amount and I lowered the column as far as I could. I'm really far back and down in the car.
2 - by the left rear main hoop support in the trim panel beside the engine lid. it's an MR2 and the battery is right there, so this gave me an ooportunity to have an external switch accessible by safety workers and a convenient place to attach the wire harness without modifing it substantially from stock where it tied to the + post of the battery.

the interior switch also interupts ignition, while the exterior swictch interupts injector power. no alternator lines, shunts to ground, etc... either aswitch kills the engine and all electricals with it. main run of cable is batt + -> interior switch -> exterior switch -> starter and power distribution