Having been an auto electrician with Hyundai/Mitsubishi/Mazda & VW dealerships I can tell you that what @DavesSpectra says is 100% correct.the service tech claimed that a DIY repair on the wiring harness would void the vehicle warranty as being "modified."
Your not modifying anything. Your carrying out a repair, just like (probably better than) the dealership mechanics would do.
And you shouldn't pay too much (any) attention to what a service tech says about voiding the warranty anyway. He's talking about something that's way above his pay grade.