Any time both lights flash, the issue is a ground. The circuit is using the second light to get to ground. To get the lights on my truck to work properly, I added a strap(wire) from the bed(bolt on the fender) to the frame. You can only see it if you are underneath. I added a wire from the mount screw to the bundle and bolted it to the frame. Covered it all up with a split tube and have no problems for the last 2 years.
