Aiieee, this was so close to being a right answer, that it hurt. They were on the right track up until they said to take out a light bulb to fix the problem. No, no no!
Here’s the deal. The trailer has to have a ground wire in the harness and a brake wire. It might also have a brake light wire and a + voltage wire.
If it doesn’t have a separate wire for the brake lights, you’re pretty much screwed. There’s no way to get the blinkers to work.
If it does have a separate wire for the brake lights, then either (a) the wire for the light and the wire for the brake are reversed, or (b) the installer didn’t bother to run the brake wire up to the pedal and simply connected both wires to the brake light.
How to tell the difference? Easy. Turn on the car, turn on the blinkers, and get out and look at the trailer lights. If they’re not blinking, you’re in luck, and swapping the two wires should fix the problem. Testing with a meter will quickly show which is the right wire for the lights.
If the lights are blinking and the brakes are going on and off, then they’re both hooked to the lights. If the connector for the trailer harness has two separate pins connected to the brake lights, then run a second wire from the brake pedal down to the connector, split the two pins connected to the brake light, and hook one pin to the light and the other to the pedal. Trial and error will show which is which. If only one pin is connected to the lights, you’re screwed and can’t use the flashers.