BAD BRAKE LIGHT SWITCH AT TOP OF BRAKE PEDAL
fuse, bulbs
brake light switch would be most common cause
The most likely cause is a bad brake light switch (if none of the brake lights illuminate) but if its only one of the brake lights, try changing the bulb. Even if they still work as running or turn lights because they have a double filament and may still work partially, but not as brake lights.
A blown bulb. Check fuses and then start replacing the bulbs one by one.
Are you sure it is your running lights & not just the brake lights? A bad brake light switch (common problem) will cause your brake lights to stay on all the time because they are not wired to the accessory switch, meaning they will work whether the truck is on or off. Hope this helps!
have you tried turning the ouside dial on headlight switch that dims the gages bright or dull had a similar problem with 98 corvette no dash lights or hvac or running lights turned out to be multi function switch (headlights turnsignals)
The BOO (Brake On/Off) Switch may need to be replaced. If it is bad, it would cause the brake lights to NOT illuminate when the brake pedal is applied.
Possible ground issue at the taillights.
Blown fuse or defective brake light switch.
Most likely a fuse.
The most probable cause is that the brake light switch is out of adjustment.
Check your brake light switch at hte brake pedal.