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The typical fuse for the navigation lights is 2 amp, 5 amp in a pinch.
I would check the bulbs, sockets, wire connections at the fixtures, wire connections at the junction box in the aft box and the wire connection at the switch and the battery.
A fuse usually blows from heat or cross connection from + to -. Check continuity, voltage, resistance, fixtures etc..... and find the problem. To keep putting in a larger fuse can be dangerous. DC fires are quick.