I have a 2000 celica just had stereo work done. I installed the stereo myself with a wiring harness
I bought from a stereo shop, no big deal easy. Went back to them they installed a aftermarket amp and still used the stock amp for my front speakers . There is no reason to bypass the factory amp when you can still use it for the front and rear speakers. The cost of everything to install the new aftermarket amp was $150 for the amp $90 to install it I have 2 kenwood 10inch subs powered by a 500 rms mtx amp that really rocks my car and sounds awesome . I been reading about people cutting wires to by pass the stock amp I just wanted to say I just used mine for the 4 stock speakers and added a mother amp for my subs easy no cutting