Tail light ground

Ok is the thing all together and wired up so as "it should work?"

If so, just get busy. Start at the fuse panel or the brake light switch. The switch should have power at all times on one terminal. Move your light / meter to the other terminal and see if moving the pedal actuates it. From there, the switch power goes to the turn signal switch. Look up the colors in the manual. I believe brake power going into the TS switch is white. Check the colors of the left and right rear. Check each of those at the column switch connector, while actuating the pedal. Should have on both those with the TS switch centered.

Make sure the kick panel connector for the rear harness is hooked up. This is yet another place you can check for power, but if you have it there at the TS switch, you should have it to the rear

Next move to the rear and probe the sockets. You should be able to turn on one signal, or the hazard flasher, or even jumper across the brake switch to get power back there.