Generally , with an AM , with no antenna plugged in you will hear nothing at all, and with an AM FM, only the strongest stations will be
audible with no antenna .
You mention only having a single rear speaker in the car . All '69 B bodies with R11 came with the single 4x10 dash speaker. A single rear was an option , and would pass through a factory rear fader switch.
If you can't get it going , that's what we do all day ,every day , so send me an e-mail.

Greg