Changing the factory keyless entry code
Enter the 5 digit factory code located on the bottom of the trunk lid. Then, press the 1/2 button within 5 seconds of entering the factory code. Following that, enter your own 5 digit personal code. This will not CHANGE your factory code. It will create an additional code that works as well as the factory code.
The only way to CHANGE the factory code is to open up the Remote Keyless Entry Module (RKEM) and modify 5 resistors mounted on a subassembly within the box. The original website detailing this procedure has been lost to the shifting sands of the internet.