I'll just jump in her for a bit. First off is your truck ever clean underneath !
But on electrical a test light would help a little. You should have power to the little parts down in the center of the socket and the outside of the socket should go to ground.
So if you ground to the frame and touch the test light to the center connection (without touching the sides, you'll just blow the fuse)) the test light should light up if you have power there.
Then going from power (from somewhere) touching the test light to the barrel of the socket should light the test light if the socket has ground.
If the socket doesn't have ground, you will have to follow the ground wire from the socket forward up the frame until you find a spot where that wire does have ground - there's your problem spot.
You can get a 20' long piece of wire, hook it to the battery, the other end to the test light and use that to search for ground.
The one ground you took a pic of isn't the only one, there's battery to frame to block, cab to frame, cab to block and a ground near each headlight somewhere, a couple inside the cab (kick panels usually) plus a few others, I'm sure.
If that ground wire coming from the sockets doesn't have ground, try splicing into it and grounding it to the frame and see if any lights come on.
Usually the ground wire will be black.

 

Agreed ^^^

But if your headlight switch is bad it could make you have power when you don't need it or no power when you do, pull the headlight switch out and test it

 

So here's 1990 tailights etc. You can see that the brown wire is common to all the things that aren't working. You said you have power to the brown wire at the socket. So if you have ground from the socket to complete the circuit the lights should light. Simple as that. You could guess a bad socket for one light out, but all of them ? Bad bulb - all of them ?
No either power isn't coming through the brown wire or it's not getting back to ground. But if your turn signals work, you must have ground ? So the brown wire looks suspect.
You can see the brown wire coming out of the h/light switch. Check it there with a test light. There it is again going into and out of the fuse box. Test it there.
Dash illumination lights should be blue and red, I think.
Best guess from here anyway. Long distance diagnosis is hard sometimes.

 

I'll have to give this a try,seemingly it should be an easy diog, (I hope) I won't be able to test till saturday,but ill let y'all know what I find.

 

I kept posting that diagram but it kept getting removed?

 

It is there on 14 post. Put a post of the negative ground splice locations if you have it. I'm going to guess this is where the bulbs in the circuit that are out are in common. I'm seeing the operator doing some twisting and taping there.

 

Idk I didn't remove anyone's post

 

14 post? I'm sorry I'm not following what your saying for some reason,please dumb it down,I'm sorry I don't quite see what your saying.

 

Turn signals and tailights are in the same socket I thought. So if the turn signals work, the socket must have ground ? That was why I was suspecting the brown wire.

 

Oh ok I got it. Man I'll be under the truck for a while probally. I going to see if there is a ground by the headlights. Other than that I only messed with wires in the back by the bumper.