Same problem here on my 78. They just stopped working mid turn one day. Hazards, brake and running lights work fine. I went through all the bulb sockets and changed the flashers, still nothing.
When I first got the truck about 6 months ago it did this. 10 seconds into determining what to repair, I felt the wires under the dash and they started working and have been fine since. It stands to reason that there must be something loose under the dash, but I have checked everything and cannot find a problem. I am stumped. Next step removing the fuse block I think.
For some reason, the power (DK BLU 38) for the directional flasher relay is branched off of the power supply (DK GRN 75) to the back-up lights on the line side of the switch:
If equipped with an automatic transmission, the back-up light switch is located at the base of the steering column - right up against the firewall - along with the neutral safety switch.
If you have a manual transmission, I couldn't tell you for sure where the back-up lights switch is located. It shows on the wiring diagram as being inside the cab as opposed to mounted on the transmission. If the shifter lever is a "three on the tree" type, the switch is mounted somewhere on the column - a little higher up than a automatic type on the NSS bracket. If it's a floor shifter, I have no idea.
Anyway, if you find the switch, check the connection where the DK BLU is spliced into the DK GRN. You should find that it's hot, if not you wouldn't have any back-up lights. If it (the DK BLU) is hot at the back-up light switch, the next place to check is the line side of the flasher relay socket.
If you find that there is power at the flasher socket (and you know the flasher is good) the next place to check is at the directional switch. The lead that runs from the load side of the flasher to the directional switch (PPL 16) is a straight shot.
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That's about all I have to offer. If the brake and hazards lights are all illuminating as designed you can eliminate the lamp sockets and also the ground side of the circuit. Also, since power for the brake/hazard lighting is carried on the same leads as the directionals, you can assume the wiring between the switch and lamp sockets isn't an issue.
The first place I would check is the line side of the relay. It's the middle of the circuit and an initial test there will tell you in which direction you should start looking for the cause.
