[BILL]

I've been thinking about your radio, I've worked on 52 Plymouth radios, they are a two piece, you remove them unbolting the center section of the dash, speaker grill, and then they come right out , nice design.
Some of these 812 had dynamic field speakers ,if you have replaced the speaker with a PM type make sure your not using the 6 volt line, this will give a bad buzz, find the output transformer secondary that goes to the voice coil and the other side to ground ,as I remember. the power supply output chassie is where the dynamic field terminal is, on the back side ,stay away from it if your using a PM.
Lastly if you got the chassie on the bench it two pieces try a jumper from the ground side of the RF chassie to the ground side of the power supply chassie