Interior lights stuck on in 06 Ford Transit 85 T260?

The fuse had blown on my interior lights. After changing the fuse everything seemed to be working ok, however, once on the road, after about a mile all the interior lights came on. Checked all the doors were all properly closed but lights still on. Tried switching lights off and this is what i found -
*The back of the two lights in the cab stays on regardless of the switch positions.
*The front of the lights goes out when front switch is in middle position.
*The front light also goes out when front switch is in forward position whilst back switch is in middle or rear positions.
Any ideas?

Gareth
October 2009