Mercedes ml 270 cdi 52 plate?

I recently purchased a mercedes ml 270 cdi 52 plate and had to change the brake light bulb on the rear passenger side. I found that one of the fittings retaining the light assembly is missing so the light assembly is loose and not fitting quite right. I have tried every where to find this part but can't get it anywhere on the net. Need a part number or description so I can search again.

Can anyone help?