Lets see how to put it together:
Step 1: Material List and Schematic
This is the schematic.
As you can see the materials needed are:
1 On/Off switch, (preferably a round key switch witch can fit in the handlebar end of your bike.
1 mercury switch
1 2.5K resistor
1 10K resistor
1 4.7μF capacitor
1 BD139 transistor, (although any NPN transistor will fit)
Also - (not shown here), 1 12V battery, (the ones used in car alarm transmitters)
Step 2: The Final Assembly
In these pictures you can see the battery used as well as the switch used. The switch has been mounted inside the tap of the handlebar. I've make the battery holder out of an old flashlight.
Step 3: Mounting
As you can see in the video in step 1, i've put the sireen inside the left end-bar grip and the wires pass under the tape and inside the tube where it meets the circuit.
Good luck and enjoy.