Brake Controller failure

I was disappointed that the tow vehicle circuit tester doesn't mimic having the trailer hooked up. The tester showed that there was no power to the backup lights. Everything else works according to the tester and the I double checked with the meter and no power to the center post. The truck itself has backup lights. I checked the fuses and relays under the hood and they all work. I am thinking about buying a tow harness for the truck. But I think I will take the old one off and look at it first to see if a wire came loose.