2000 GMC Jimmy Tail left running light, and Top brake light

The most common cause of the problem you describe is, someone has spliced in a trailer connector in the wires by the hitch. Most of these trailer kits come with blue squeeze connectors which actually cut into the wires. As far as I'm concerned, they should be federally outlawed. If you have one of these, that's the first thing to check. If you don't, you need to test for loss of power or ground at the lights to see which you are missing. Brake and tail lights are on seperate fuses, so you can have one without the other. Hope this helps.