I just replaced my non-JBL speakers with a set from Focal. They have kits specifically designed for Toyotas so no adapters are needed. On the Gen 2 Prius you need the Focal IS165TOY 2-way component kit for the front doors, and the IC165TOY 2-way integrated kit for the back. Now I have wonderfully clear sound - at around the same volume level as before. I wanted to post here to help others, as the installation guides I found on this site gave me the confidence to do this. Thanks to everyone that posted before.
