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After reading Smitty's post and thinking about it for a moment, I wish to retract my pinched wire theory, at least tentatively.
When the brake lights illuminate when the headlights are off, are they quite dim?
The only explanation that I can come up with as to why you would have brake lights only when the headlights are off is as follows: The brake and running light sockets share the same ground in the tail light assembly. If that point in the circuit is not grounded, the current from the brake lights will pass through brake light bulbs to the socket ground, then backwards through the running lights, up to the headlights, through the headlights, and then to the headlight ground. It may find an easier ground in the headlight relay when the lights are off, I'm not sure about that. At any rate, the power must pass through at least one extra set of light filaments, and this means that there is a great deal of resistance in the ground path, so I would expect the brake lights to be dim. When you turn on the headlights, the brake lights cannot ground through the running lights any more, because the current hits a 12V live wire on the other side of the running light bulbs.
The question is: How did working on the tailgate wiring disrupt the ground to the tail light sockets? Did you do any other work, or pull hard on the wiring harness above the headliner?
The suggestion that the ground wires in the hinges have failed was good. However, even when those wires fail,the tailgate should ground through its hinges and latch. It is not a good idea to use moving parts as ground connections, but the electrical stuff in the tailgate will all work properly. The reason that you need the ground wires is to avoid the corrosion and sparking that occurs when you use moving parts as electrical connectors.
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