The premise for the lock was that when you locked your doors, the gas filler door locked as well, and vice-versa.  It is a simple concept where a motor extends a pin that enters a hole in the filler door, deadbolting it. 

In 2003 & some 2004 model vehicles, the Tech II programming is already there, so when you lock the driver door, the fuel door locks as well.
Some time during the 2004 MY, this was changed to an option (or the default was made to "disable") and for some 2004 & all 2005, it must be enabled in Tech II.

The tech must, through Tech II, add the BCM module, and then activate the Lockable fuel filler lid. See picture below.
Info courtesy of NickTaliaferro.

I don't know if all this applies to convertible or not, but would assume it does.

Installation instructions courtesy of 93_newbie.

Took me about 30 minutes but I could do it in 10 minutes now that I have done it one. The steps are as follows.

1) Remove the protective cover in the trunk. You can remove the circular attachment clips using the nub on the handle of the screwdriver. Push in the middle of the button clip, remove the clip & place somewhere safe.  Be careful as the clip is in 2 pieces and can easily come apart.  I only removed the top attachment and simply folded the cover down into the trunk.

2) One the trunk liner is removed, you will find another liner that underneath that has three attachments. Two round plastic disks and one wiring harness. Push the center of the disks through to remove and simply pull straight out on the wiring harness attachment and it will come of the threaded screw is sits over. Remove this liner piece completely.

3) You now have access to the inside of the fender and to where the lock is attached. You will need to with your finger push the rubber seal that seals the lock hole currently in the edge of the filler door out (it almost looks like there is a hole through it where the locking arm could go but this is not the case...it is sealed so either push it though from the back side with a finger or pry it out from by the filler cap).

4) The parts you purchased are the lock mechanism and a rubber protector/gasket. The rubber gasket has a pointed tip to it. This is actually only there to help you feed the the gasket through the hole and it rips off when you are done. Only do this after you have pulled the gasket through the hole by threading it from inside the fender. I think that the best way to do this is after you have the gasket seated on the plunger arm and the lock in place.

5) Slide the accordion gasket over the plunger arm on the lock and make certain that it seats over the locking lip at the bottom.

6) You will see that the lock goes into the fender with the heads of the screws on the bracket sliding into the holes on the fender side into the trunk. Take the locking mechanism and extend the plunger fully. Find the lock hole and guide the tip of the gasket into it and pull it through the hole into the fuel cap area. You will see the gasket seat. Then screw the bracket and lock into place. Rip of the tip of the gasket by the fuel cap.

7) Cut the zip tie that holds the wiring lead. There are actually two leads attached together. The other one may either be for a CD player of a trailer harness not really sure but I will venture that it is for a CD player. Run the cable through the hole just above where the wires are attached and snap the lead onto the back of the lock mechanism. Provided that you have ripped off the tip of the gasket in the last step you can now test it with your remote.

8) Twist tie the other wire lead back into place & replace all the trunk cover pieces.

9) Have a beer or coffee depending on time of day and tell your wife how difficult it was to do and that you need to relax for a while.

S=Standard for this model

O=Option available from the factory

R=Option not available standard or from the factory, but is retrofittable.

blank=Option not available standard or from the factory, and is not retrofittable. Or info not available.

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