Break out the voltmeter and"
1) check for voltage at the nav lights. (yes or no)
If no go to 2. If yes, check the bulb and internal connections.
2) Check for voltage at the "load" (light) side of the NAV/ANC switch with switch set to NAV. (yes or no)
If no go to 3. If yes, there is a wiring problem between the switch and light.
3) Check for voltage at the "line" (input) side of the NAV/ANC switch. (yes or no)
If no, go to 4. If yes the switch is bad.
4) Check wiring from the fuse/breaker panel to the line side of the switch.
5) Repeat 1-4 for the courtesy lights
You said you looked at the fuses. Did you check them with an ohm meter or simply replace them with known good versions.