Trailer light plug does have turn signals just brake.?

I have a 4 wire trailer light plug. White, brown,yellow and green. I put circuit tester on the plug and neither the yellow or green wire show any activity. I grounded it to white. When I hit the brakes it shows activity in the yellow and green. The cord runs into a panel in the back of the truck. It's a 2004 Hyundai Santa Fe. So I really have no idea where to go from here. Any suggestions appreciated.