Why not spend $1200 more ($3000-$1800) and get a new Garmin GTX-335 Transponder? Yes, there's a few more hours of labor of course, that also includes longer term transponder reliability and not having to fly with the nav lights on all the time. Converting the Nav lights circuit to Nav lights/ADS-B Out circuit on the same switch would make me a little nervous.
I know there are a lot of pilots who only care about the absolute price. For others how are you managing from an ops perspective? Putting a placard over the nav button, carry extra bulbs to more frequently replace due to earlier burnout from constant use? Curious.
(Sorry for the thread highjack)