I had this problem when I bought my van at 8 years old. This is a design problem so it’s very likely that others have it too..
The problem is caused by a Water Trap behind the front locker / front bulkhead. Water gets thrown up here from the car when towing in heavy rain…
- Treat the affected (and as yet un-affected arear) under the floor with Cuprinol 5 Star. In fact, why not go along the WHOLE of the underside on both outer edges.. See the staining and mould there… it’s getting damp!
- Then after a couple of weeks to dry… Spray the area with Waxoyl… Yes that Waxy stuff to stop cars rusting underneath.. NOT Underseal!!!!
- Spray into all the nooks and crannies especially behind the bulk head.
- Better still – use “brushing” Waxoyl and apply copiously… don’t worry about it running down your arm… it feels horrible, but it will wash off with some detergent – not so easy for your clothes though!
- Then a couple of weeks later – put some more Waxoyl on!
- I also recommend buying some Universal Mudflaps (Ebay!) for around £12, cut these to fit and fix them just behind the wheels by screwing a wooden batten just behind the wheel arch with the mud flap fixed to it using steel repair strips (B&Q). – and they apply Waxoyl.
I can upload photos of this if you really want!