thanks, hocky, for your useful comments.

i since did some research and found out that the rubber plug is where the washer motor for the headlights would slot in!

i guess the plug has developed a leak and i am going to buy a replacement from the local dealer tomorrow.

i also found out that fastlane sell the plugs for less than a dollar and there are some good pictures of the plug on the fastlane website.

only i live in the uk, so it will be quicker and cheaper to get it from my local dealer.