Radio questions and speakers

Not sure which forum thread the Radio falls into but....
I have the original AM radio, which is not coming on. I suspect it will work, but right now the entire dash panel is not working. 1966 dart 270 with 6cyl 225

My question is to the speakers. Is there a wiring chart for the radio and speakers? I see one speaker in the dash, which appears to be completely rotted out. Then on the back seat shelf there is 3 speakers, which are all new. Speaker cables have been run to the back.

Is the dash speaker a normal size that can be replaced?
Also I am curious as to anyone who has attempted to upgrade the radio? Like added a FM or a iPhone input somewhere? CAn it be wired into the existing radio (for power and spekaer runs)