Just wanted to post an update. I took a closer look at the tail lamp bulb sockets. Keep in mind this is a new harness. I was able to turn the round contact disk inside the sockets with a small screw driver which effectively reverses the signal to the bulbs (which only fit in one direction) -problem solved. I think the harness manufacturer installed the contact disk backwards, but it's a super simple fix. Thank you for the suggestions, it helped solve the issue!