I'm pretty sure that the 02 RT10 headunit cannot be replaced with a regular aftermarket headunit. You have two choices:
1) Keep the stock amp, and splice the aftermarket unit wires to it.
2) Ditch the stock amp, and use a separate amp. On my setup, I took out the stock amp. My aftermarket headunit had a built-in amp. I then spliced the wires to the existing front speakers. For added bass, I installed an Infinity Basslink (similar to a Bazooka tube in that it has both the amp and sub built together). Sounds GREAT! ChrisGTS also installed the same Basslink in his O2 GTS. He kept the factory amp and headunit, though. His Basslink just pulls a signal from the factory setup (whereas mine pulls the signal from RCA cables attached to my aftermarket headunit).