I looked under the front passenger seat today to confirm that I don't have an external amplifying unit under the seat.
I didn't see any rectangular silver box or any wires, so I guess that means my Supra has internal amplifying.
There was a hollow rectangular black flexible thing that was under the seat though (where there was a opening cut in the carpet, near the front of the seat, next to seat bracket bolt closest to the transmission shifter area). I have no idea what purpose it served. But definitely didn't see any wires or silver box. Did not have a camera on hand to take a picture.