I have had turn signals (from Time To Time) Since I bought my 1975 Bronco. The wiring in the back is a mess. Periodically, I have had turn signals for no apparent reason during this time. I put new flashers in it right after purchase. Didn't fix it. Then one day without me doing anything, They started working. However at this time, I had no brake lights and only one running light. Today I bought the new stop light switch and after fixing some shorts on poorly tied together wire in the back, I have brake lights and both running lights on the rear. (No marker lights or tag light yet). However, I still have no turn signals. Up until last week I could hear the blinkers clicking when I turned them on in either direction. Now its back to nothing. I tightened up both brown wires going for the running lights, and am left with 3 wires. There are plenty of little connectors with one wire coming in and 2 going out. These are where I have open wire. On the RH Stop and Turn signal lamp feed I have a green wire coming into a connector and two going out. One is connected to the left side light socket and the other is broke off danglingwith no voltage. On the other 2 connectors, one looks black or brown and has 12 volts on it. The other is a green and yellow it looks like with no voltage. This thing has had a healthy dose of undercoat some time in the past and they got the wires extra good. Short of the new rearend harness, any ideas? Also I have nothing in the front for turn signals either. And here while back they were working and I tried the flashers, and it made them quit again. Honestly I don't think they have worked since then.
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i would try the wire harness in your sterring column my 77 f-250 had the same problem but i went in and just say old cracked plastic and my buddy said that thay wire harness might be bad so i got a new one and works like a charm it might not work for you but it was a sugestion

 

I don't have a diagram for your year truck, but it sounds like some of the wire colors are the same.

The brown is for the running lights. This wire feeds both the taillights, and there should be a splice in this wire for the license plate lights.

You should have another wire that goes to both sides, and it will be a color(possibly a black/red). This will be the back-up lights.

Then there are two other wires. In 1976 they were green for the right brake/turn and lightblue/black for the left brake/turn.

I do not know if you realize this or not, but the same wires that carry the brake signal also carry the turn signal. So if your rear brake lights work, but your rear turn lights do not work, it is very possible that your turnsignal switch has a problem, since this is what handles all the signals from the flasher and the brake switch, and mixes them all together.