If you could draw a wiring diagram or sketch, showing the handlebar switches, the flasher can, and which wires have voltage, wire color, the lights themselves, and anything else useful, it would help to come up with something that would work.
Make one sketch with the existing wiring, and a separate sketch with what you want.
If you don't have a scanner, try the Windows Paint program or something similar, save the sketch as a jpeg file, and post in a message.
It is important to have a good idea of what the parts and layout are before you
try to add something to a circuit.