Go to Radio Shack (or similar) and buy two 1N4004 Diodes $1.19 Each, a 12 Volt Beeper or Buzzer ( P/N 273-055 for $3.59 @ Radio Shack) and a bit of Shrink Tubing.

1. There will be a silver stripe at one end of each Diode. That's the "Cathode" end of the Diode. The end furthest away from the stripe is the "Anode."
2. Connect the Cathodes and the Positive Wire for the Buzzer together. Solder and Shrink Tube work real well.
3. Connect the other end of the Buzzer to Ground (Frame of the Bike).
4. Connect the Anode of one of the Diodes to any one of the wires that is for your left Turn Signal Bulb or Indicator Lamp Circuits and the Anode for the other Diode to one of the wires for the Right Turn Signal Bulb or Indicator Lamp Circuits. Again, Solder and Shrink Tube work well. Bare wires are NOT good, make sure all wires and connections are covered.
5. Your Buzzer will now turn "On" and "Off" with either Turn Signal Bulb Circuit (The Buzzer will cycle "On" and "Off" with the Bulbs, not the Turn Signal Switch.) and the Diodes will keep the two circuits separate from each other.

The whole thing will cost you less than $6 or $7 depending on how much you spend on the Shrink Tubing.