The flasher is inside the truck, roughly behind the radio. I doubt that it's bad because it is shared by the hazard circuit, which you say is working correctly.
The turn signal switch in the column may be bad or one of its wires may be chafed inside the column. The green wire to the switch would be most suspect.
Another place to look for a chafed wire would be the red/yellow striped wire to the hazard light switch.


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