hi. you will need a n/s mirror .manta b,cavalier mk1,ascona b.
this theoretically can be fitted to the existing corner piece (wont sit correct without some mods )but better to obtain the correct corner piece with the mirror indentaion/hole biult into it. someone on here will no doubt be akong to offer you one. or pop an ad in the wanted section.
to fit it involves removing the door trim complete and unbolting the corner section with runner.
there is one on ebay at the moment and the picture shows clearly why youre better off with the correct door mounting.and what you would need to shave off if you didnt .