Everything is already there. All you need is the switch, the solenoid, catch, and the door itself. I got all the parts from a junked 93 and installed it in my 89. Whole thing took about an hour. Hardest part was getting the plug out that they filled the hole in the filler neck area with. It's glued in pretty hard. Electrically speaking it all plugs in and works.
Seems that it was planned to be an option in 89 but got cancelled for some reason.