Above and beyond testing the wiring harness from the engine to the key switch, it is good practice to install a small, but heavy duty distribution bus near the battery(ies) that is powered by the "accessory" position on your selector switch. From this bus run #8 or #6 positive and negative wires (depending on the number and draw needed for power accessories to a similar bus behind the console. From the console bus, run power to your fused switch panel for all you accessories - VHF, GPS/MAP, fish finder, navigation lights, windless, etc, etc. In this manner, you will always have a power source at the console for testing and adding additional accessories.