The lights were not drawing enough power to trigger the flasher, so I have to add a 10K resistor in parallel. I did it by taking apart the lights and adding the resistor inside the socket (I like to keep the bikes as stock as possible and I don't want to make any mod I can't quickly revert). I can post picture if there is any interest
BTW, kudos to Ernie for this post. Before finding it, the "sneak path" drove me crazy
