Found this how-to on a dutch forum actually. So here's a quick translation for you guys! (sorry if this a re-post)
This is an easy mod for this type of tail lights:
So the problem is that the single reversing light is no longer white. However, the indicator glass is still clear and could be used both as an indicator and reversing lights. This way you can do a fun mod: install TWO reversing lights and TWO rear fog lights!
What you need is a small round led light that you can fix on top of the indicator light holder:
On the holder of the reversing light, solder the red and black cable of the led rings:
It should look like this now:
And the final result (he only did one side):
I'll give this a go myself if I can find the right parts!