diff lock light

For the past couple of months my diff lock light has been coming on intermittently whilst driving normally on the road. It gets worse especially when I'm turning left.

Whats causing it and how do I fix it?

Landy is a 110 CSW with a 200 TDi, retro fitted.

Can anyone out there help?

http://www.landyzone.co.uk/photopost/showphoto.php?photo=7102 You can just see the switch under the high/low selector, has the wire come off or worn through and is it earthing out on summat or is the switch nertled.