2004 Peterbilt 387

I'm having some issues with the lights in the bunk of my truck. I got power in the front i can use my gps and the radio but don't have any power in the back. Sometimes it does work for a while and then i can hear like a wairning buzzer coming from the bottom of the bed and then i loose the power in the back. Any suggestions where i can start looking would be really appreciated.