Just keep in mind, OEM navigation offer following two functions likely is not offered with aftermarket navigation. They are important to me:
1) SVC (Speed-Sensitive Volume Compensation). No more audio volume change when driving. It goes up & down automatically along with speed of the car. Three sensitivity levels plus off for SVC. It is possible since Navi received speed info from E's ECM/PCM vehicle speed pulse. Our E is not the quietest car by long shot, this function allow me to enjoy music, regardless speed, w/o manually changing volume, so I could pay attention to the road.
2) GPS will work w/o clear view of sky when on the road. It continue track E's location on the road in the GPS when deep inside the forest or in the tunnel. It is possible since Navi received speed info from E's ECM/PCM vehicle speed pulse, forward/reverse info from E's reverse signal, & Navi unit's own build in yaw rate sensor to determine E's orientation. I don't know how many time I was really appreciated of this function, when I was going through confusing back roads in Santa Cruz mountain here in North CA w/o stopping.