Do you mean the Powertrain Control Module, (PCM) for the engine, the Electronic Automatic Transmission Controller, (EATX), the Body Computer, (CAB), Air Bag Controller, or Antilock Brake controller? The engine Computer is under the air filter housing behind the right front head lamp. The Transmission Computer is on the left side. The Body Computer is behind and to the right of the glove box.
If you replace either the Engine or Body Computer, AND your car does not have factory installed anti-theft system, you MUST install either a new computer or a used one from a car that also did not have anti-theft. If you install a used computer from a car with anti-theft, it will teach that programming to the other computer, then both of them will require the disarm signal before they will work and let the engine start. At that point you will have to replace both computers for the engine to start.
If your car does have factory anti-theft, you can use any computer. If the replacement isn't programmed for anti-theft, it will learn the programming from the other computer as soon as you turn on the ignition switch. Also, if you borrow a computer from a friend's car as a test, and your car has anti-theft but his doesn't, your car will teach the programming to his computer. After that, his computer will no longer work in his car. The anti-theft programming can only be upgraded to work with that system. It can not be unprogrammed.