I have not done this but I have considered it and I think it can be done. The best place I can think of to tap in (with the JBL audio) would be at the amp under the passenger seat. On the small connector the 2nd through 5th wires from the right side (facing the amp from the plug side) are the input wires. Black is Left – White is Left + Green is Right - Red is Right + I think just taping into these wires would work provided that you had some sort of a switch mechanism to disconnect the existing input. Let me know if you want to see the wiring diagram.
I believe some people have, based on this website: http://densionusa.com/merchant/merchant.mv..._Code=IA_Toyota http://www.densionusa.com/x/index.php?opti...d=375&Itemid=79