The switch is broken. Common fault. Once it leaks, it should be replaced or it just gets worse and worse over time. The wire that plugs into it has a rubber seal around it, but that's to keep dirt and moisture out of the electrical contact.
Fortunately, they're cheap (well under a tenner, even at a Vx dealership) and easy to replace in two minutes with a spanner, hand tight. They're all pretty much the same so its ok to buy a cheap no-name one from ebay or parts retailer as long as it fits.
If the electrical plug is soiled with motor oil, spray it down with WD40 and wipe it off before plugging it into the new switch.