I had this problem as well after my spoiler got nicked. Searched around all of the car shops and ended up finding some blanking grommets from Repco in Osborne Park.
Only problem on the gc8 is the hole that the wiring goes in to isnt circular like the other ones. To combat this I cut two smaller grommets in half and glued them together... a bit agricultural but worked a treat lol stopped the water getting into my boot