The OEM (stock) head unit (HU) on the Gen 3 package II Prius leaves much to be desired. To get iPod and Bluetooth support, one must upgrade to a package III with additional options costing several thousand more dollars. For this and many other reasons, many Prius owners choose to purchase a minimal base package II, and upgrade the head unit themselves.
This page introduces general aspects of head unit upgrading that apply to any brand or model of head unit. Click on each tab to view the related material.