It has to be mounted on a level surface(or very close to it) with the top of it exposed to the sky so it can read the GPS signals.Does it not have to be strapped down in a certain direction for the accelerometer to work properly? I assumed, and the OP may have as well, that it is directional specific and needs to be mounted, kind of like a trailer brake controller.
Sunglass holder won't work, or if it is taking readings there, it can't be the most accurate (poor GPS reception and not level)!
An ideal place would be under the front or rear window. Just that I'd have to use velco or something, there is no metal object there....... On my GTO I could stick it to a speaker grill in the rear and it worked perfect.
I'm sure you can also mount it outside on the fender or roof or something, but I'm not risking it flying off.