The horn and nav lights don’t share a circuit. So I believe you have 2 separate issues,if everything else electrical is working. Your best bet is to take it to the dealer as it’s under warranty. That said, To get the nav light apart you have to take the whole housing apart. check if you have 12 volts at the connector under the deck, before taking it apart. If you don’t have 12 volts there, take it to the dealer, there’s a problem in the wiring. If you do have 12 volts, the fixture needs to be taken apart to see if your getting 12 volts there. Could be a loose connection, corroded connection, broken wire or just a bad fixture or bulb. (Mine was a loose and corroded bulb contact. After all that, I had to replace it a year later, same issue) As far as the horn goes, check the circuit breaker, check to see that you have 12 volts at the switch and if you do, if you get 12 volts at the horn when you push the switch. Depending on what you find, could be a bad horn, bad ground, loose connection, broken wire or something else. So, like I said, take it to the dealer and let them chase all that down.
Hope that helps.