You'll need to find a 65 Tbird parts car that had the rear speaker, then remove its rear package tray (along with bracket and bolts for the rear speaker). After painting it to match your interior, you should be able to install it with no problems.
If you have no luck at your local junkyard, call one of the Tbird vendors that carry used body parts. Guys like Pat Wilson
www.wilsontbird.com, Jim Dottling
www.thunderbirdconn.com, and Nick Pepe
www.t-birdsbynick.com, have parts cars on hand and might even have a package tray in the correct color that won't need repainting. Good luck.