I recently bought some electric mirrors from ebay to convert mine. The door plate on one was not as good as mine, so I thought I'd swap them.

Took the plate off of one of the old mirrors first, and it was a nightmare. They push fit onto the hinge which has a retaining lug that clicks into place when the mirror is pushed onto it. Ended up cutting the old mirror up to get it off, which obviously doesn't help when fitting the new mirror as I'd have to cut that up to get it off its existing plate.

Ended up keeiping the one from Ebay