Reasons for Aftermarket Stereo:
- Better Sound Quality (?)
- Aux in (iPod, DVD player, etc)
- Built in Bluetooth handsfree
- Aftermarket stereo harnesses, on some models, can destroy or be incompatible with OBD Readers.
- Solution is to cut the K line so that power is not sent over the line (traditionally the power is sent to extend powered antennae)
- Article for more details