i have two rear speakers in my 66. i have like this black heavy felt like stuff an the speakers are mounted on top of that material an nuts from under in the trunk. wire go forward under the sill trim plate to the radio. but i have no pictures to show you this install. the original front speaker is still there but not wired.
Zoom in on the rear deck you can see them. Have car 42 years & still remember cutting out those hole. Heavy gauge metal not like the crap toady. lol Take your time measure 3 times if needed. Did the speaker come with a templet ? You can find one on line. Highly recommended! Job comes out better & it makes it easier....Hope this helps