make sure the wiring harness is corrosion free, apply small layer of dielectric grease, and be sure the harness is securely fastened. You may want to also make sure that the tail lamp (bulb) is securely installed
I believe the F has and LED tail lamp not a bulb. I would check the tail lamp connector under the fender and make sure it is tightly connected. Terminal 1 should be the power to the tail lamp and terminal 2 should be the ground wire for the lamp. If the ground is bad you will probably notice the same issue with your turn signals.