I can't specifically tell you where you went wrong, just give an educated guess, but it is highly likely you have a bad ground. That said, the first thing I'd do, just to be safe, is use Circuit Tester # PTW2979 to ensure you have the proper wires connected, as going by color is often not reliable. But generally when both turn signals flash there is a bad ground on the trailer.
You'll want to check the ground area for any paint, corrosion, or other build up and ensure the ground is attached... view full answer...