Nakamichi Replacement Help!

[92'SC400]

Ok i have read countless threads and can't seem to find some kind of guidence to go about replacing my factory Nakamichi radio with an aftermarket one im so lost its not even funny. I have put afetmarket radios in countless times, however when i took the one out in my 92' SC400 im left scratching my head. Now i know that there are stock amplifiers in the car, ive replaced the door speakers, the two in the rear deck and the sub it sounds great but the CD changer is goin out on me and i want to upgade anyway. So the radio is out and there are i think four different plugs in the back of it......My question is can you buy a harness for the nakamichi setup or am i out of luck, and when you buy a harness where do the rest of the plugs plug into since the stock radio is going to be gone. And is it possible to just put in a aftermarket radio and still keep the stock amps in place and working. Im to the point of just putting it all back together and dealing with it. Please someone help pics would be awesome too....actually anything would be great. Thanks for the help in advance.