Miles -- if you are referring to your TR4 then note the above may be referring to the TR6.
TR4 wiper motor housing was usually painted, possibly some variance in color used (wrinkle balck, hammerite grey, etc). The door latches on mine are a natural metal -- I see no evidence of cad or other plating, but that could be the age of the piece.
Sort of depends on what type of restoration you are going for... concours, flash or just close to original.