Yes, it is possible to add an amplifier to the stock Supra setup. You will need a line level convertor and about the best place to receive this signal would be the factory subwoofer. Simply disconnect the wires that go to the factory sub and wire those wires into a line convertor. From the line convertor you will then run RCA to the amp. Run your power wire into the inside of the car via the yellow airbag grommet on the left hand side of the car. (It is up high and to the left of the dead pedal) Everything else is fairly explanitory and should you have more questions, post again..
Thomas Kenyon
Custom Sounds
San Antonio, Texas
Oh yea, make sure you get a line convertor that has an attenuation on it. PAC's SNI-35 is a good choice.