Most cars have a push-in switch located above the brake pedal, and when the pedal is depressed the switch is released and turns on the brake lights. This also cancels the cruise control if it's on. I don't have a Murano handy to look, so...
Get down under the dash on the driver's side with a flashlight. You'll probably have to run the seat back and lay on your back.
Press the brake pedal with one hand, and use the flashlight to scan the entire brake pedal unit as it moves. I'll bet you can spot a switch that is opening. If it's out of place, you may be able to adjust it. If it's moving properly but not working, the switch needs replaced.
In the meantime, keep the car on a battery charger tender to avoid having the battery discharge.