Reset Keypad

I had to replace the battery in my keypad (door unlock), and it has since not worked. A buddy of mine said you have to reset the system sometimes. How is this done on a 1999 Mustang?

i didn't even know you could get a key pad on a mustang!

i didn't even know you could get a key pad on a mustang!
*slaps head*
Im an idiot....
The thing on the keys, not a door keypad. Like the remote-unlock. W/e it is called. lol.

bump plz help.