Trouble shooting 3rd brake light

[ElevenB]

Hey all, any help would be appreciated here. My 3rd brake light flashes on when I start the truck up but does not work at all when stepping on the brake pedal. The two cargo lights work just fine. I cannot seem to find a solution to this anywhere on YouTube or elsewhere.

 

[Loudram]

Are they LED or incandescent? If they are incandescent have you checked the bulb to see if it's completely seated or if the ends took some heat. Check the plug connection as well. Always do the basics first.