Right rear tail light not working, help ASAP

[xxvxx]

I need help trying to figure out what’s going on with my right rear tail light. I have a 2014 gmc sle I upgraded to the 16-18 denali led tails. So I had to buy a harness to help it not hyperflash from gen5diy. So my right rear signal doesn’t work and when I press the breaks. I assumed it was the aftermarket harness with the resistors so I bought new resistors from gen5 installed them and still the same problem. I found the schematics and found out with wire was for that right tail light. Also found the fuse. When I was testing the fuse with a Circuit tester pen that fuse is flashing fast like when is hyper flashing so I track that wire and when I go to test out that wire the circuit pen doesn’t light up so there is no power in that wire check the wire from front to back everything seems good so I need help because I don’t know what else to do. And now out of no where my headlight drl is not working on my left side either and both of my actual headlights don’t turn on “low beams” my headlights are the 16-18 denali. And also now my left rear tail light is not turning on but when I turn on the signal the left rear it work! 
please I need help, I’m going crazy here. I feel like I’m missing something simple