When it comes to Blue Sea, make sure you are making a distinction between a conventional dual battery switch and dual circuit battery switch. There's a big difference in application and Blue Sea makes both.
Tough to beat a ground close to the engine block grounded at the engine block (the largest metal mass on the boat). But if there is any resistance between this point and the battery ground point then you have a voltage differential and therefore a potential noise conflict.
I don't know what the ground pin is but the outer shield is a breeze to identify once you snap the DIN plug end apart. Clarion technical should be able to give you a pin diagram. The ground pin is easy enough to identify with a basic multimeter.