Trailing light wiring
 

Hi,
I have a 2000 Chev Express Diesel with a wiring problem!! I have no brake lights at the end of the trailer wiring harness inside the truck. Is the circuit independant of the rear lights on the truck? All the lights work on the truck. All the lights work at the harness except brake lights. Does anyone know if a seperate lighting harness is run for the trailer harness or is it spliced into the rear lights on the truck? Does anyone have a wiring diagram for the truck? Thanks for your time in advance

Matt