The Toyota Prius comes with a fine audio system, regardless of whether you choose the stock system or the optional JBL upgraded system. An additional amp comes into play when a subwoofer is added because neither of the supplied Toyota/JBL systems has a subwoofer as part of its component system. The installation of an amp can be quite easy, though, because the battery in the Prius is located under the rear seat. There is room in that space for an additional amp and subwoofer, and the proximity to the battery makes installation and connection simple.
Place an external amplifier beneath the rear passenger's seat, close to the Prius' battery so no holes will need to be drilled to wire the amplifier to the battery.
Secure the amplifier in the rear seat compartment with double-sided tape or industrial-width Velcro.
Wire the speaker level output of the Prius' JBL amp to the external amp's input. This retains the audio system's volume, fade and balance control. You either need an amp that has speaker level inputs, or you need to convert the speaker level signals to preamp level signals.
Connect the positive lead (+12V) from the battery to the amp and ground the amp to any bolt under the rear seat. Both wires should be fairly large gauge; follow the recommendations in the amp's manual.
Add an inline 1-amp fuse to the 12-volt power line to the amp. This ensures against burn out and overheating.
