Wow. That's weird. The 1st video to me is strange because it's only the center marker and not the other 2 that blink. Over all I still think there is a bad ground, but crossed wires is a possibility. I'm not impressed with the ground the 3 markers are hooked to, it looks rusty. Matt may be right too, it could be under the hood. Unplug the headlight relays and see if that changes anything.
Maybe start eliminating lights to see if it changes, turn on the signal and unplug them 1 at a time, then the upper taillights. If it changes, start adding lights back in, skipping the one that changed it.
I think you need to get back to basics and start unplugging the things you added then go forward again 1 thing at a time.
I wish I was closer, If I could be there I could probably figure it out pretty quick. The internet is great but doing this type of thing is hard,