You can get a new remote mirror for a 68 Cougar here:
http://static.cougarpartscatalog.com/2001052.html
It is a bit smaller than the Bronco and the bolt pattern is shorter. I have a pitted Cougar remote in a parts box that I can photograph next to the Bronco's remote which is mounted on my Bronco if anyone wants to PM me with their email.
The Cougar's remote has a slightly different base than the Mustang. Cougar's is T shaped while Mustang's is L shaped.
My 68 Cougar has an original remote mirror on it.
I lucked out on the Bronco remote mirror as it looks brand new...off eBay a few years back for something like $50. Problem is I can't locate the longer plastic gasket. Tried to cut an old inner tube.....FAIL! Anyone know where I can score one? If not, I will make one from black plastic, I guess.