Adding a 3rd. brake light

[2001 Glide Rider]

Hi Guys,  I'm adding a 3rd brake light to a friends 2011 883 sportster and my problem is the light that I mounted to his fender flashes with the signal light. All else (tag, tail and brake work fine) but the brake light flashes when the signal is on. Any help at all will sure help me. ps If someone would please draw me a diagram it would make it so much easier for a dummy like me. Thanks in advance.........

[andyxlh]

Sorry dont know for sure, but could it be that the flasher is somehow earthing through the extra brake light? Did you run a feed from the brake an an earth also?
On my old bike, there are 3 wires to the taillight, one is the earth (black) and then 2 colored wires. One runs the brake light, one the running light. If you run your light from these you should have no problem, one to he earth and one the correct hot wire.