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When you say that they "blink" do you mean they do this in relation to engine RPM? Or all by themselves?
The lights changing intensity generally does not indicate a battery problem but more an Alternator or Voltage regulator problem. I bet the lights do not "blink" when turned on but with the engine off. If so then I would look for a connection problem.
Do the lights brighten when you increase the idle speed? A little may be normal. You can take your Alternator to a parts house and have it checked, usually for free.
Put a voltmeter on the battery and see if the voltage is changing there. Also see if the fan belt is tight enough. I have seen them slip a little when the lights are turned on.
My 1969 F250 was doing the same thing. I had the electrical system checked out and found the voltage regulator was bad. Bring your truck to a place like Autozone or Checker /CSK and they should have a machine they can check it for free
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