2000 blazer trailer lights

My blazer has a towing package so the wiring harness was already there without the plug. I picked up a 4 pin plug and a led tester. Used the wiring legend that was attached to the harness. When tester plugged into 4 pin plug the stop lamp LED lit which it should but the 2 for the turn signals did not with the blinker on. These 2 LEDs light up when the vehicle is off and the brake pedal is pushed but not when the vehicle is running and the brake is pushed. Also the stop lamp LED does not light up when the brake is pressed with the vehicle off. It doesn't make sense to me and I am driving 3 hours to pick up a trailer. I want to make sure they are working before I get there. Any help would be appreciated.

Sometimes the polarity will be off a bit, throwing an LED test plug out of whack. Use a test *lamp* to figure out if your lights are working properly. I had the same problem with a tow harness on my Dodge, polarity was all screwed up from the *factory* no less. But plugging in a lamp to test it out, rather than an LED plug showed that indeed, everything worked.