So the old trim got broken from old age it seems. I've ordered the replacement trim but it's proven much harder to fix.

Is it true that there is a special tool for it? 

been a while ...i think you seperate the lighter socket parts , push the plastic trim in from the front then slide on the metal parts from the rear?

You were right

The green plastic had to be separated first

Because the old trim broke completely I couldn't see how it was assembled in the first place