Your amp should be under the drivers side dash to the left of the steering column. Remove the three cover panels underneath to access it. If you want to replace your radio with an aftermarket, the wiring harness that plugs into the amp (long, thin rectangular one) will need to be physically re-routed (REALISTICALLY IMPOSSIBLE!) or extended (yes!) to reach behind the radio.

Another caveat: The harnesses are coated with this disgusting slime lubricant at the factory to aid routing without damage. It never dries. Wear surgical gloves when touching them, and plan on changing them a few times. If you don't do that you will be washing your hands every 30 seconds.

I sold my orig amp & radio on Ebay. There is a market for these things as long as you have the radio code.