Cheers to Tiny for pointing this out. The plastic nuts holding the arch liner in place are the same as the nuts used to hold the rear lights in place. If you lose one of the rear light nuts you can borrow one from the centre of the arch liner until you get a replacement.
Replacing the lights on the new smart is a whole lot easier and quicker than before.
Open the boot and look to each side, you will see a flap on each side.
The offside flap (shown below) hinges forwards when the clips are pushed in (circled red).
The nearside flap is larger and hinges downwards but has the same clips.
With the panel removed you can see the rear of the light. It is not totally necessary but unplugging the wiring connection will help with access later on.
Squeeze the plastic clip (circled below) and the bulb holders can be pulled out on one part.
You have now accessed the 3 bulbs, these are bayonet type that
have to be twisted to be removed.