High mounted brake light comes on with headlights

Try removing the tail light bulbs, then seeing if the problem is still there (I bet it isn't). Then try the bulbs one at a time to determine which side is causing the problem. Then, swap the bulbs to see if the problem stays with the side or with the bulb.

If the bulb is doing it, replace the bulb. It will either be the wrong type (single filament instead of dual) or will be internally shorted by a broken piece of filament. If it stays with the side, look at the socket connection to the shell of the bulb and the ground connection to the socket. A voltmeter or test lamp helps, but it can be done without.

Mike

Check The tailgate harness were they go through the hinge for breakage

To expand - the power wires for the 3rd brake light and license plate lights are in the same harness in the left hinge, they can short together there. Also, if the tailgate becomes ungrounded the licence lights can steal a ground through the brake light filaments. (There are wires in each hinge to ground the tailgate, in addition to the hinges themselves as well as the latch, so this is unlikely, but I've seen it.) And as the original responder said, the double filament bulbs in the taillights can have the 2 contacts on the bottom join together.