Where is the factory amp on a '93 Celica?

Please help me out, im trying to install a new head unit and i need to plug the wiring harness in, it wont fit in the peice that the old head unit came out of, i asked best buy why and they said i need to follow the wire to find the fac. amp. but i can not follow the wire, i dont know where it is going. please help me, thank you so much.