Help with replacing trim....

Hi - has anyone tried to replace the triangular trim between rear passenger door window and quarter panel glass (black gloss piece)? For some reason the piece on the passenger side is fine but drivers side is badly corroded, I have taken a look but cannot work out how it comes off????.

Also can anyone recommend where I can get a replacement (tried A8 parts but they can't help - apprently its a common problem so the cars they break always have corroded pieces).

On a related note, I'm also trying to find a replacement passenger door mirror. The unit is fine but the chrome cover is scratched badly - searched and searched on-line with no luck (also tried A8 parts but no luck), so any help appreciated (along with clues of how to replace - I'm a complete novice to the world of DIY car repair!!)