Ever mount speaker in defrost hole??

There is an oval 6x9" hole in the pass. side wheel well trim panel. That is where the blower motor mounts in a Barracuda fastback. I'm just not sure if it is the same dimension as a 6"x 9" speaker. You're 73-76 duster/dart should have an electric rear defrost , not the blower style. My 73 dart sport did.