Has this ever worked in your truck? Did you check the fuses? Have you checked for codes? Do you have an ABS light in your dash? High mount 3rd brake light work? Cruise Control work? When you connect a trailer does the DIC say TRAILER CONNECTED? There are lots of problems with this system and knowing some of these answers may help solve the problem. One relatively simple remedy is to replace the trailer brake system relay located on the rear frame above the axle. Another is to replace the trailer brake pressure sensor on the master cylinder.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator