The front fender side marker lights were used on '68 Cougars, Montegos, Comets and Cyclones. They are not the same as full size Mercury.
The same light assembly was also used on '68 - 72 Ford pickups with the Camper Special option package as the bed light. Look in the truck section of the junk yard, the light is mounted at the top of the cab above the rear window. The lens on Camper Special light assemblies is clear rather than frosted so it is not correct for Cougars. Not many people would notice the difference unless you were to point it out though.
Full size Mercs in '68 have a big rectangular side marker about the same size but it is different. If you saw a picture it would be pretty obvious. I am travelling and don't have access to a lot of things right now.