That looks like it's been there for a long time. Before you prise it off, take note that it is already cracked and maybe won't survive being taken off , so I'd get a spare first.
Looking from here, I don't think it's that anyway. It looks from here as if you got something that I'm gonna have to sort on mine very soon. There seems to be a bit of crud under the windscreen seal, and that is a favourite spot for it. Very very carefully, lift the rubber up and have a close inspection. The steelwork and paintwork there has to be perfect. If there is the tiniest of rust bubbles there it will bugger up any chance that the rubber has of sealing, and once its started it lets more water in and sort of holds it in too just to be a real bitch. Try and hold the rubber up with something that won't damage it, possibly the ends of some clothes pegs and be a bit careful if there is rust as you might press it into the rubbers.
To prove it, can you see it when it drips ? To prove it, firstly take your pegs out then seal all around the wiper where it touches the body with cling film and or blue tack, seal around your fresh air vent too. Now make a tiny watering can with an old milk bottle with a biro body as a spout, pop a little hole into the bottle neck about an inch from the top, get a biro body that has a thread on it, usually has a plastic bit on the end that people chew, the bit unscrews / comes off to get the innerds in and out. Screw this into the bottle then fill with water and put the top back on. The idea behind this is to be able to direct the water precisely but keep it to a small amount. Squirt it at the rubber for roughly thirty seconds right where it joins the body but just do about two inches for thirty seconds, then go and check in the van, keep going around the rubber to body join and if you get nothing, then go around the rubber to glass join. It might be right in the corner or a top corner and running down inside the rubber. If there is nothing initially then try it with a large watering can to try and make it leak, then if it don't then clear your spindle protection and try that, or your air box but do them one at a time.
Ozziedog,,,,,,,,,,, try and track it down.