i have a 2013 gmc sierra with a service trailer brake light that comes on at start up.i have replaced the dash switch, the module under truck n had it programmed, light still comes on , now 700.00 dollars later i'm lost, any help?

Check for stored codes.

Check wiring at the brake controller module.

Check the trailer connector(*) for corrosion etc.

Check trailer wiring (in the truck) for damage.