1996 duetto revisited

Following the big rust repairs of two years ago I've had two seasons out of the van and was hoping for a third in view of the amount the repairs cost, however, I discovered about two months ago that the carpet in the driver's footwell was damp.  Underneath seemed to be dry so I wasn't too concerned but last week I noticed it was still damp and on the underside too, and the van hadn't been out for about three weeks.  Further investigation found that the carpet under and towards the back of the driver's seat is soaking.  I then remembered that I had on occasion heard a drip round about the driver's door during the night if it was raining though there was nothing to  be seen in or near the door itself, so there must be a hole or something somewhere and the water is getting inside the panelling.   I am wondering where is the most likely place rain would get in?  There is nothing obvious but I haven't looked at the roof/guttering properly yet.  Has anyone come across this in a Transit?