As a passenger in a truck or automobile, you are already receiving quite a bit of electromagnetism from the engine's electromagnetic field, especially if it's a hybrid vehicle. Most readers are already aware that reflective surfaces will increase your electromagnetic field radiation exposure, due to the ricochet effect, so why would you ever want your cell phone set on cellular and add to this electro-schmutz?

Not every vehicle has a built-in navigation GPS system. So, what do you do if you are lost while driving? Yes, you can use you phone's GPS if you have location services activated. Just set your phone on "airplane mode." 

In airplane mode, your phone is not sending/receiving electromagnetic particle-waves, benignly termed "signals," from the nearest microwave transmitters (A.K.A. Cell Towers). With location services enabled, your phone's location is being tracked by satellite and the GPS app will still work. There is still some increase to the already present EMF field, but less than when the phone is on the cellular setting.

This is all a compromise; using the lesser of evils in a pinch, but you will be able to calmly find your destination and avoid distractions that could cause an accident.