Have you verified all the lights work properly in the rear first? I.E. get someone in the car, turn on the parking lights to verify the running lights work, then step on the brake lights as well. If they light up fine, then we need to do some troubleshooting here.

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards