1990 Nissan DoorLock KEYPAD REPROGRAM??

Hi,
I own a 1990 Nissan Maxima with the Keypad at the doorlocks. I didn't run it for a while,after i bought it used and it lost all of the information on my keypad. I can';t reset it because i need a master code to make myself a code. How can i reset the numbers so they work again? Any help apreciated.
Ryan

I almost positive the code is located under the passenger dash. Just go under there with a flashlight. I think it's hidden behind a black box.