I have signal brake and light failure at the plug for my trailer that I pull. on 1999 Ford F-250 Super Duty

The plug has power to its right side (brake, signal, running lights) but there is no power to the left side, could this be a relay problem? I have also checked the first plug under the bed of the truck,(this is the plug that supplies power to the tow plug located on the bumper) and there is no power to the left side at that point. Are there other harness plugs I am missing?