If you're still getting a check trailer wiring message, then there has to still be short somewhere in your vehicle. If they fixed the ground and wiring for the trailer then that's only solving one half of the problem. The other half is with your vehicle, and since you're still getting the error we can confidently say that it's your Chevy's wiring that the problem.
So, to find out what's going on you're going to need to go over your wiring and look for any broken, crimped, or damaged wires.... view full answer...