"Service Trailer Brakes System"
Had the exact same thing. No power to the trailer brakes.
It was caused by an ABS module.
Mine went bad the end of July 2016.
There were no replacement parts in the entire nationwide GM inventory.
It wasn't until January 2017 I was called to say they had the part.
Back in August 2016 I ended up having to bypass the factory controller and installing an aftermarket one. I needed power to my trailer brakes.
After spending a couple hundred dollars and my time to hook up the aftermarket controller I haven't bothered to go back to Chevy to get the factory one back in service. It was going to be around $500-600 for parts and labor.