Hi I have a 2001 gs 430 car, I am trying to locate the stock amp, i want to put a4 chanel amp in my car, i also is upgrading my door speakers, and also putting another amp to run 2, 12 inches subs i am not changing out my head unit because i have the mark leviston navagation system and i dont want to mess with it, so could someone tell me the location of the stock amp. also is there a way to keep navagation system and change out the radio unit and replace with a aftermarket radio if so could u all direct me B)