I'm nearing the end of a long F100 project, and I'm running into issues with the turn signal wiring. When I started, the brake and taillights were out, but the turn signal would light both rear lights at the same time. The front turn signals work fine. So I decided to run new wiring to the rear of the truck. It used to be that there was a green wire going to the left, a yellow wire going to the right, and a purple wire that ran to both from a homemade electrical tape loom from the PO.
I looked up a schematic for the signal-stat 900 mounted on the column and ran wires in ENT that seemed to vaguely correspond with what was there: One for the right signal/running light, the same for the left, and a wire for the brake light. I started doing some test runs, and suddenly nothing works. I tried all kinds of wire combos and got nothing. I pulled a wire out of the cab and touched it directly to the battery: nothing. Then I thought maybe the wires had shorted in the conduit and brought the bulb and housing into the cab and hooked it up directly. Again, nothing. And yet, the front signals still work fine. Occam's razor is telling me I must have blown the bulb, but it worked prior to the start and I've tried both filaments and they both look solid.
There is a very good chance that I'm missing something quite stupid and simple as my car wiring experience is not all that great, but I'm really stumped over here.

 

Never really paid much attention to them till I had to swap 'em out on my 64. The wiring is the same 61-66, except the length depending, long bed/short bed.

Notice three wires come in, four wires come out. There's a common hot wire, just like you say?

 

I just noticed a note in the instructions that said not to wire in the turn signal cam until the end. Is there a chance I blew out the front half of the cam or somehow blew out the rear portion of the turn signal unit by wiring it hot?

 

My working theory is that the tail light gets wired in to both the yellow and green at some point, basically making the light go on permanently when the headlight switch is on, and the turn signal's intermittent current would make it blink when engaged. Then the brake lights get their own signal, ran in series from the new switch. Am I in the right ballpark? And there's no dedicated ground for these lights. Right?

 

The more I look at this the more I'm convinced that the issue is grounding the rear taillight assembly. Any tips?

 

Yep. When I ground the dim bulb tester to the new wires I ran it lights up. This tail light assembly isn't grounded (or there's a possibility the bulb isn't making contact). Does it just ground to the body? I tried wire wheeling a bit of the area behind the tail light assembly and the back of the tail light assembly to no avail.

 

You can run a dedicated wire from the negative battery post for test purposes. But there's no need.

The ground path is through the frame and body. Make sure the ground straps from block to firewall and block to frame are bombproof - clean, bright, and shiny tight. Use new cables and tighten securely. Coat with vaseline or NO-OX.

A test light is useful. Disconnect the harness at the firewall to remove any possible issue with the body harness portion, see there's only three connections, right?

Turn the key to ACC and then turn a blinker on (left or right, try the other later) and ground test light on the engine block. So now that's four.

Remember there's a common hot wire for the running lights, that splits off later on in the body harness. Then the one each, for the blinkers.

 

Okay, so I solved my ground issues, but now I'm having function issues. I've got it to where the signals work, but when I have the left blinker on and hit the brakes, the right brake light won't go on and the left blinker stays solid. Strangely, the right is fine with the brake light on. found this wiring diagram, but I can't get the picture insert to work, so here's the URL. I should be in figure A, but I'm having trouble reading it:
signal_stat900series_zps0c3bce7c.jpg Photo by _ogre | Photobucket
Is the brake switch really open on one end? Is there a wire just going between the two lights? Are those arrows diodes? Does this mean I need an entirely separate wire for the running lights? I've seen a different diagram without the wire running between the brake light switch line and the crossover line.
I'm going to try a few things tomorrow and pray to a variety of deities and see what sticks.

 

Sounds like poor grounding. The frame, engine block, and body ideally are all at the same potential. Like an electrical bus. The whole truck is the negative ground plane.

The way to kinda tell bad grounding just by observation, remember while electrical voltage is applied, current is drawn.

So consequently loose or missing/corroded ground connections will cause a current hungry device to suck up the current it needs through alternate even though longer path.

A headlight with a lifted or missing ground might draw it through turn signal hardware or connection. So bulbs stay lit, but won't blink while headlights are on, etc. It's a real bugaboo with vintage iron, and even sometimes on newer rigs. Rust and corrosion won't conduct. Restored trucks often have similar issues except it's usually caused by thick layers of paint.