We have never had a problem with leaks being reported in these windows, so it is probably worth checking a few other things first before getting involved in removing and replacing them:
1. The sliding portion can not be 100% waterproof as it has to allow the glass to actually open - but the system is designed to drain out any ingressed water directly from the lower rail - there are two drainage tubes that expel water from the rail out to the bottom edge of the window.
2. If the drain hole is blocked then this water can't escape. So it is worth dribbling a bit of water into the lower rail to check it can get out OK and the passage is not blocked
3. If the van is parked pointing up-hill then any collected water in the rail will flow to the back and should still drain out. However if the end stop is not sealed properly some water can dribble out the back end of the rail.
4. otherwise the main reason for water coming is an incomplete seal where the van and glass are bonded together - and that's down to ensuring a continuous run of sealant with no gaps when you stick it on. Unfortunately there's really no way to fix that other than removing the window and re-sticking it.
The only other thing that can happen is that if the foam seals are very dry they do not seal as well as if they are a bit damp, and therefore softer. It is worth just running your finger around the seal with the window open to ensure that they do not feel 'brittle'.
I'd be 99% sure that one or other of these will sort your problem but if not please just get in touch.