I'm not sure I understand "...erratically..." The frequency and/or period [length] of the on and off cycle[s] make a difference in the analysis of cause. However, I'll illustrate a couple more possible causes. Generally, the headlighs are on a "dedicated" circuit, all by themselves. This case suggests that the problem is in the headlight circuit, and is why the running lights are not affected.
1. A LOOSE CONNECTION anywhere in the HEADLIGHT circuit, from the battery to the filaments in the bulbs, can cause "blinking." If the blinking DOES NOT occur when there is NO VIBRATION [as when the vehicle is standing still, and the engine is off], then a loose connection is the more probable cause. Another suggestion that a loose connection is involved is that the blinking seems to "be in time" with vibrations of the vehicle. A corroded ground connection, or actually corrosion in any connection is falls in the category of a "bad," or loose connection.
2. For many years, some manufacturers have used "automatic resetting circuit breakers" for headlight circuits instead of fuses. The purpose was to reduce the hazard of totally and completely "loosing" headlights during total darkness. When a fuse "blows" out, the lights are completely out of service until you stop the vehicle and replace the fuse.
With an automatically resetting breaker, when it "cools down" [usually within a few seconds}, the headlight will come back on momentarily, and continue cycling that way, giving the driver some time to slow down, pull off the road and stop.
If your flickering involves time periods of a second to two or three seconds on and off, it is possible that there is a SHORT CIRCUIT in the headlamp circuit, to which the circuit breaker is reacting.
To locate the short, look for a headlight circuit wire "pinched" in a small crevice between two sheetmetal panels, or insulation "chaffing" on a sharp bend around a sharp edge of sheetmetal.
The only way you can completely turn off the headlights is to push in the emergency brake slightly, then start the truck and leave the emergency brake on the slightest setting, and the running lights will stay off. Obviously the brakes will be slightly engaged, but you can adjust the emergency brake so the first click doesn't engage the brakes.
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