In the case of the cell phone the antenna needs to be parallel to the direction toward the source.
The best place to mount the detector is as high as possible where it won't be obstructed by other objects. Behind the windshield wipers is a bad idea, but near the top of the windhield and ahead of the rear view mirror is a good spot. You need to consider how much you want to detect reflected signals coming from the side or rear you want to capture.
Mounting on top of a 6 foot mast (right next to the radar seeking missle
) would probably improve detection range, but would also attract a lot of unwanted attention...