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I am not familiar with the electrical lay out for your Montauk but my guess is you have a dash panel with switches that also act as circuit breakers. I personally am not a big fan of that arrangement (personal choice) because tricky to match breaker with additional accessories (among other reasons). If you have a panel....... is it hot wired to the battery or do you also have a 40amp or 50 amp breaker in line? If you go with an additional switch, wire it to the inline main service breaker. If you do not have the service breaker do install one in case you have issues with your switch panel. Either way you need fuses for the radio and GPS but I assume that you have inline heavily jacketed fuses on the + wire to the units. A good idea if you install the switch to keep it in the off position when starting the motor to prevent "spike" damage to those accessories.
Garris