Tail Light Issues

[Sammy]

Check all the connections by pulling each apart and reconnecting them cleaning starting with the dash harness, neutral safety, flasher unit, harness to headlights and to tail lights. 90% of the time always it's a bad ground or a bad connection from a splice.
"Now however, my tail lights don’t work. They light up when my blinker is on and when my head lights are on, thats the only time. only my left back up light works as well."
Fleetside tail light housings bulb connections are a common corrosion area. The cross over tail harness gets alot of abuse from previous trailer connections and the elements. Good Luck and post the final fix.