Attach an electrical wire to it by means of a threaded machine screw through an electrical crimp fitting and the other end to the engine .
Make sure there is very good continuity at the attachment points, ie: clean any paint etc off and attach to the clean bare metal. I believe it should be aluminum.
If its something like this in the link, you will see a screw shere the earth ot ground needs to be attached to...
http://www.ebay.com/itm/32mm-Car-Wat...-/232169975933