2012 international 4300 no marker lights or tail lights.

These are powered directly from the body controller, no fuses. Once the body controller detects over current it will kill the circuit if you have a short. To reset it, simply perform a key cycle. If you have a broken wire in the main supply, this will also cause no lights, but not likely because it would mean a break inside the cab. Over or under current both will log codes to help diag what sort of problem you have. Most of the time it is short and most of the time it is the wiring to box lights or tail lights that are rewired by the body builder. Also, if the truck has a lift gate, check for wiring that may be crushed and shorted out due to that. Disconnecting the wiring section by section is the best way to diag. Hopefully the body installer used connectors for the rear connections so you don't have to cut possibly good wires to see where the short might be. The wiring also likes to rub through on the hood and bumper area. Factory wiring will have connectors to the hood near the headlights at least so you can disconnect those one by one. Each time you disconnect something, remember to turn your lights off, key on, then key off, turn your lights on to see if stay on or keep going out. Good luck!

 

Water from leaking windshield dripping down into fuse box ?
International is famous for this... Those relays turn green and grow inside when they get wet..