Nah. What you're wanting to do isn't too complicated if you take it a piece at a time. But first a question:
Will you be activating the turn signals, brake lights, and headlights when the wagon is horse drawn?
The headlights only when being horse drawn, not when towed. Brake lights will be flashers only with horse, no turn signal with horse.
Brake lights, turn signal, and electric brakes used when towed, just like any other trailer
I would like to put switches on head lights, and flashers. Brakes are used to slow horse down, kind make it work harder and tire faster so cooperates better, not to stop the trailer. Harness and horse do that unless you live where it is hilly. I don't. Not worried about drain on battery for it won't ever get used but a few hours at a time. Parades, hay rides, and fun around the area. No trail blazing or long distance camping. So I will charge the battery as I need it.
Brakes are exactly what you said, axels and hubs are trailer type, P235-65/15 tires. Sewing machine doesn't hum with no pressure, but spins faster as you apply more pressure, why I thought it would make a good brake pedal.
Was going to run wires from battery to termination blocks with a kill switch between, from the blocks to switches to corresponding items. Then wire the trailer plug like any other trailer tying in the lines in between the rear light assembly's and termination blocks (scotch loks, gotta love em). With the battery kill switch in the off position, and the truck hooked up, will I get any feedback to the blocks? And will that hurt anything? And where do I tie in the brake line?