You want it firmly attached to somewhere flat. I data logged accelerometer data (not from an LMA-3 though), and found inside the center console was the flattest place in the car. I'm not sure how useful 2-axis data are though... any deviation from flat will throw off the numbers due to gravity, as can vibrations (depending on the sampling rate and filtering of the accelerometer signal). You need 3-axis to properly determine the 'up' vector and the true longitudinal/lateral forces.
