One problem with the F/TFs is that the fuel filler cap tends to fade and corrode over time. There are several options including painting them etc, but I just bought a good second hand one. The problem with this is that the replacement fuel cap will have the wrong key which means 2 keys on your key ring. You can however swap your lock barrel onto the new fuel filler cap. If you remove the cap and look on the inside you will see a blue plastic shroud.
You need to remove this as it holds the key barrel in place. If you look at the end of the blue shroud you will see that there are 4 quadrants which clip under a raised section on the end of the lock barrel.
You need to bend these quadrants back far enough that they release their hold on the barrel.
And then pull the shroud away from the filler cap.
You will then see a rubber o-ring which needs to be removed.
Once the o-ring has been removed, hold the cap and push on the cream coloured part of the lock barrel so that you push the barrel through the cap and out towards the key end. The o-ring is still in place in the picture but should be removed.
And the barrel is removed. Now remove your barrel in the same way and push it back though the new filler cap. Don’t forget to fit the o-ring back onto the end of the barrel.
Reform the quadrants on the blue plastic shroud so that they are roughly the same shape and position before you levered them and then push the shroud onto the end of the barrel until it clicks into place. You can only fit it on in one of 2 ways, make sure that you have it in the right position before pushing it home. You can test this by turning the key and making sure it operates properly.
You have now changed the lock and can fit the new filler cap to your car.