1995 Chevy Truck brake light problems

hello, i have a 1995 chevy truck. my problem is i used the truck to pull a trailer last week and noticed the rear brake lights don't work, the third brake light on the cab works but not the rear lights. i changed out the brake light switch and no luck. i am leaning toward the turn signal switch but they are expensive and i want to make sure that is the problem. i am a mechanic and good with electrical but i don't have a schematic of the light system on my truck. i only have one power wire coming out of the brake light switch when the peddle is depressed. any help would be appriciated. thanks mike
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