[Tommy]

Mike,
You say the brake light is dimmer on one side than the other. Is the parking light perhaps brighter on that side than the other too? If so, the wiring to the two filament bulb is reversed. ... You can quickly check the poor grounding theory by running a jumper cable from the tail light ground to a known good ground. If the light gets brighter, you've found the problem. If not, it's something else.