2002 dodge 2500 p/u diesel when the headlites are on the running iltes & brakelites don't work. With the headlites off the brake lites work

It could be a ground issue
Check all bulbs,make sure they are the correct numbers ,and check ground wire from taillights /parking lights to frame
Check that third brakelight on the roof also
You could try to wiggle the sockets/ground wires at night to see if it all of a sudden works
If you have any modifications like trailer wiring,then check the ground wire there first

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