Rear Sidemarker - 1969 C10
 

My truck came with only original sidemarkers front and rear without lamps. In redoing the truck, I had the front markers replaced with the lamp versions and wired. I replaced the rear sidemarkers with the lamp versions, but have not wired them up yet. Can these be fed into the wiring harness that carries the tail lights and rear bumper license plate lamp? Have not looked, but obviously there has to be some sort of original harness to carry those wires to the front power source. I am an amatuer at these sorts of things.

Yes, run a wire to the tail light harness, tap into the brown wire, that is the tail light wire.