This just may be the circuit you need. This is my bicycle trailer taillight interface circuit, a logic "OR" demonstrator, and bike trailer that contains it.
It has been in my trailer since day 1 ( April 4, 1987), and has worked flawlessly since. The PCB is designed around the PC-mounted relays. The 'reverse' circuit is optional, included for my Grote taillights.
I didn't use the larger Bosch relays or a steering column switch assembly then. You can adapt this circuit for 6V positive ground.
In my jurisdiction, horse-drawn wagons and carriages need safety triangles at the rear wall or frame. This trailer will eventually get one, after a major overhaul.