Interior light switch only works with ignition on

I've got a 1996 Mk3 Golf 1.4L and have a problem with the interior lights.

The problem is when the ignition is on and I open the door the interor light comes on and off when I close the door. When the ignition is off the interior light stays on even when the door is closed.

Any ideas on what the problem might be?

I think if you wait, the the light will eventually extinguish.Try locking the vehicle,it should extinguish straight away.

I have tried locking the car and came back 10mins later and the light was still on. Any other suggestions greatly appreciated. Cheers