1994 Nissan Truck Tail Lights/Brake Lights

My tail lights work find whenever I turn on my headlights or running lights. However, only the high brake light works when I hit the brakes.

Despite the fact that the tail lights work fine as tail lights, I replaced both bulbs and all the applicable fuses, on the basis of "maybe." Neither lower brake light comes on, only the high brake light.

I have the electrical and automotive knowledge of a small puppy, so please use small words in any suggestions. (I know that I should simply take it to a mechanic and let him take care of it, but I'd like to know what it might be and how to fix it before doing so, if only to keep a new mechanic honest.)

Thanks.