I looked your 2015 truck up, I think the easiest place to make this modification is at the body control module. I am not sure where it is on your truck, they seem to move it around after every few years. But it should look almost like a fuse box, it has fuses in it.

On the body control module, fuse F30, 15 amp and fuse F40 10 amp are the two fuses that feed the marker lights. You can pull these fuses to verify it is the lights you want to modify. The way I am looking at it you have two choices;

Try to modify these two circuits with some sort of fuse tap rigging in the fuse box or

Find the yellow/green and the yellow/blue wires going to these fuses and cutting those wires and modifying that with a double pole switch and two flashers.

 

Thanks Dave. I'll see if I can locate them and give it a shot. Thanks for the guidance.