1995 Jeep Cherokee Brake Lights won't function properly

Check the brake light switch. See if it has power going in one side sand when depressed do you get voltage on the other side of the switch? If not check the fuses. When you turn on the headlights both tail lights and brake lights work?

The brake lights are on a seperate circuit than the headlights. Check the wiring by the fuse box. Did you say you have voltage to the brake light switch with the luights off. If not then there could be a crossed wire, you should have voltage to the brake light switch with out anything on.