If you have the amber/yellow rear side marker covers, the biulb holders are blanked off. You need the red marker like the USA type. You need to visit you Mercedes Benz part centre and buy two bulb holders. Also you need two bulbs 501 type 5w. These are same part as our front parking lights. You can wire these though the rubber gromet to get to back of rear light unit and wire into rear lights at each side. I did this mod and its a big improvement.

Good luck