Originally Posted by Denny
I would apply a Diode to keep it from back feeding to the lights.
Good call Denny! I was going to suggest the same thing!
Try putting a 1N4001 Diode ($1.50 at rat shak for two) on the horn relay activate line..should be the small black wire.
The Collapsing field on the relay could be inducing a forward Bias on the diodes as the relay opens and closes, causing them to "Blink", It only takes a small amount of voltage to light them off.
Also be sure none of your wires cross that or other close circuits under the dash, give it as much separation as possible.