Wiring question...

[HumbleChevyBuddy]

When installing a third brake light license frame, I see three wires on the back of it (white, red, black). There were no instructions with the kit, but I'm assuming white is ground, red and black would go to the brown wire in the rear. It all works well when I turn the head lights on, but when the lights are off the third brake light won't come on at all.

Can someone think of something I'm doing wrong? Could it be a wiring issue with my truck? I also noticed that the left turn signal flashes REALLY slow when the head lights are on. However, when the head lights are off the left blinker flashes just fine.

Any ideas?