I think I've tracked down the source of the buzz to the amp.
I disconnected the radio and the buzz continued.
I disconnected the cassette tape player and the buzz stopped. Most likely because the speaker wires run from the tape player, not the radio.
I connected both back up and then disconnected the amp wires. The noise would only stop when the mail plug was disconnected. I'm assuming because that one must be the OUT TO SPEAKERS wires (including the power and ground wires as well.)
In the picture below is the amp, mount just below the cd changer.
I've circled the plug that makes the noise stop and have arrows pointing to the other 2 plugs.
NOTE: unplugging the main plug not only stops the noise from buzzing but it also cuts off the amp so there is no sound from the radio or tape player as well.
