I've got a 65 with AM radio...but I can't find any speakers anywhere!

Nothing in the upper middle console, or between the back seats. Those are the only 2 spots I know to look though. When I replaced the carpet I found a long white OLD speaker wire on each side coming from the center console and going under the back seat, but they were loose on both ends.

Anywhere else I should look?

The front speaker mounts in the center of the dash. If you lower your control panel for wipers and vents you will see it. If your car was equipped with a rear speaker you will see a vent in the rear package tray(where the bird emblem is) and thats where it mounts. Non rear speaker cars just had a package tray with no grille in it. If you fold down the rear armrest and remove the inner black metal panels you will find it.

Nothing, and nothing. There is no "grill" between the rear seats where the T-Bird logo is. Would there be a "grill" in the center of the dash?

From what you say then your car was a single speaker car. The only grille in the dash will be the grille over the defroster vents also. Lower your clock/vents/wiper panel and thats where it will be, unless someone removed it. If I remember correctly the last one I changed had 4 wing nuts holding it in place.