Redman,
your voltage thing with the motors is not specific to crowns design, all motors can do it, especially if they have brushes. Even brushless A/C motors can do it if they get enough dirt in the field windings.

As for your Motorola radios picking up voltage through the mount?
I'd say the radio was intended to be grounded through the mount so you could isolate that by going into the radio and moving the ground from the mount to a separate ground that would connect directly to negative, of just removing it all together, it's probably a secondary ground point. But your solution of isolating the mount plate is probably the simplest solution.