I think the cable i use is 6 ft. I put it in a separate box - is an electrical wiring water proof box with a sealed lid - and mounted it in the middle of the cab right above the center of the differential. Yes, right in the dead center of the pivot. Putting the Brick on the roof I find makes it quite useable.
What you want from the brick is angular velocity of the tractor so if its mounted more to the front of the cab away from the pivot you get a lot of extra motion (sideways) when turning. That negatively affects the gyros and accelrometers