Look at your schematic that came with the harness. Study it to understand how hot gets to the rear. Then see that it is wired correctly. Start at tail lights and see if you find hot with a ice pick tester. Be sure you touch the tester wire TO A GOOD GROUND. Grind a new place on frame for a good ground so the test will show whether there is hot at lights. That is the first step. Then work towards front to find hot. Don't assume some screw or bolt is ground. You can also run a length of wire from battery ground back to the rear temporarily. Take out bulbs, turn on lights and touch tester probe to each of the socket contact pads while this temp ground is attached to tester lead.
I have it in my mind that turn signal harness has 7 leads???

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn