Turn signal beeper. Credit for this goes to bierdo@dirtly.com. The following is merely a demonstration and is not recommended or endorsement by this website.

You'll need:
Wire cutters/strippers, wire, solder & soldering iron, some heat shrink tubing and a heat gun.
Bridge rectifier (Radio Shack #276-1152A - $1.25) & Piezo beeper (273-060, $3.29 NOT the same one shown in the photos)

A. Assemble your tools in a neat area.
B,C&D. Spread the wires of the rectifier out & tin the ends. Note where the positive & negative wires are.
WARNING: Don't get the rectifier too hot or you will damage it!
E. Strip & tin 4 wires, both ends.
F. Solder the wires to the rectifier
G. Shrink wrap the open wires to prevent them from touching each other.
H&I. Shrink wrap the entire assembly to help weather proof it. Do Not overheat the rectifier.
J. Solder the positive & negative wires of the rectifier to the corresponding wires of the beeper.
K. Secure the remaining wires to the positive leads of each turn signal. One to each signal, doesn't matter which one goes where. Mount the beeper in a safe spot and secure the rectifier. Make sure the steering does not interfere with the wiring.