You can connect two wires - one from the blue/red and one from the blue/black - to the black/white indicator wire. Only one of the wires - like, the wire from the blue/red - would have to have a diode in-line; this will prevent the flow of current from the blue/black side to the blue /red side. I think, just to be safe, I would put a diode in each line. Now your indicator bulb will flash when the right or left turn signals flashes. Many things are so simple, but, with extra effort, we can make them very complex.