Most likely turn signal flasher
Either the driver is indication the wrong way or the wiring to the indicators is crossed.
have you checked the fuses?
either a bulb or the signal flasher relay located on the fuse panel usually opposite the hazard flasher relay
A short in the wiring.
Most likely poor wiring.
Loose connections or worn parts in the light switch.
You may have a bulb that that brunt a element which than crossed the other element cause all lights to come on a one time. I have seen this before. If this does not light all bulbs when the brakes are applied, then I would be looking in the front lights, if it does I would look in the back lights.
Either your battery is dead, or your alternator has failed.
1. Bulb 2. Fuse 3. Flasher 4. Turn Signal Lever
The whole turn signal arm unit needs to be replaced
The lights wiring is grounded and you might want to check to see if the ground is connected properly to the frame.
First thing you want to check is the dimmer switch to see if it is turned all the way down.