The OEM amplifier should be behind the glovebox. It may not be a simple plug-n-play with a harness adapter, as you may have to extend and hardwire to the wires that connect to the amp. I did this on a '98 Avalon with the similar methods, leaving the amp plug that have the speaker wires to the car unplugged to the amp. This, of course, disabled the factory sub also. There is an old thread (or two) somewhere that could shed some light. Try look for them first.