GMC Troubleshooting Questions? Ask a Mechanic for Answers
hi . Check on the rear cross member to see if your truck has the IC lighting module
okay check these fuses under the dash
the module at the back of the vehicle is where I would check for power on the plug coming from the front of the vehicle . To be honest the standard practice when a vehicle does not have a conventional " pickup box " on it and a flat deck as in your case is to wire the flat deck lighting to the trailer harness
if there is an issue at that connector ( no voltage ) you may still have possibly have an issue in the multi function switch ( turn signal switch ) but check at the rear connector first .