The retainer that holds the keypad in is still available new from Ford..You can go to your local Ford dealership, and have them order one for you..
Here's a link to one for sale online-->>KeyPad Retainer
All of the 1990-1997 Thunderbird & Cougars used the same retainer to hold the keypad in..
I guess 1989 was the oddball year..That retainer is only available NOS if you can find it..
The Ford Part Number for the 1990-1997 MN12 KeyPad Retainer is: (FOVY-14A667-A) or (FOVY14A667A)
The Ford Part Number for the 1989 MN12 KeyPad Retainer is: (E3SZ-14A667-A) or (E3SZ14A667A)
Here's a diagram of the KeyPad & Retainer:
MN12 Keypad & Retainer Diagram.JPG
Not pictured in this diagram is the gasket for the keypad that is sandwiched between the keypad, and the hole in the door where the keypad sits..
The gasket is no longer available from Ford..If you're interested I'll provide the Ford Part Number for it anyways..
The Ford Part Number for the 1989-1997 MN12 KeyPad Gasket is: (E9SZ-14B061-A) or (E9SZ14B061A)