You can get them from motorsport auto I think, but with postage etc perhaps we just get the parts from jaycar/dick smith? It is just a plug that fits in where you would normally attach the voltage regulator. Inside the plug you put a little diode and connect the pins to make the loop/connection. Then fill the plug with resin to stop it vibrating and finish it off. Plug might be a couple of dollars, diode might be a few cents and I have the resin. I could make up a batch cause it seems like a few people need them...
http://www.viczcar.com/forum/index.php/topic,785.15.html
http://www.thezstore.com/page/TZS/PROD/12-4067