It's not in the manual or maintenance guide or other papers at all. If you're lucky, the original owner put a sticker somewhere - on the manual or guide, or on the trunk lid hinge. This was common in the 80's and early 90's. Later, the stickers went away. Therefore, the only thing you could do is look at the module or plug in to the module.
The factory code is located on a sticker on the remote anti theft personality module (RAP), which I believe is in the drivers footwell area, above the accelerator pedal. Alternatively, and more conveniently, a diagnostic tool can retrieve the code from the module.