Does it happen only when the van's being driven or when it's parked up?
Any work been done on the van around where the grommet for the main wiring loom comes through the bulkhead (behind the glovebox) like an alarm fitting or a power cable for an amp?
Leaking windscreen seal.
Blocked drainage below the landing panel under the wipers.
happens when parked up. Water is very clean. Garage says been around the windscreen with a hosepipe. half a bucketfull in each well. Will look at your suggestions,Ta