Looking at a first transit -and yes its a rust question....

[Beaker]

Water off your boots......
Thats either coming in through a leaking windscreen or down the A piller via the stupid trim clips that hold the external plastic trim on.Either way it will find it's way right down to the inner step as well as sitting under the floor mat.
You have to lift the plastic floor mat and undo the two 10mm nuts under the step that retain the plastic step covering and have a good look at the condition of the metal. You will also have to get down and dirty and check underneath . If it's been left unwashed underneath and laden with salty mud then it won't outlive your trusty hi ace......
The chassis construction on these MK 6/7's is abysmal and with the drain routes designed to exit through the front of the sills the inner box sections are quietly rusting away unless you flood them with a runny waxoyl mixture.
Check it over forensically.

That's an interesting idea - I never even thought of that. I've definately not got a leaking windscreen so I might remove the A pillar trims and have a look.

Any idea how to remove them?

They simply prise off . The clips ,or most of them , will break so best get some replacements before you remove them. Like you i was certain the windscreen seal was ok but then I had to have it replaced and the fitter showed me two 1.5 inch sections that were passing water. I ran a solid bead of pu sealer along the top of my screen after discarding the plastic trim fitted and again sealed off those trim clips after finding water ingress under the drivers A piller. Ihad ignored the damp under the floor mat for a couple of years and ended up having to cut out two areas and weld in fresh metal.
Since all above was done there has been no reoccurance of water getting in.
I cannot emphasise how important it is to get under the, or any,transit to check the floor and chassis. Mine has had the box and sill cavities flooded with waxoyl every year inthe 13 years I've owned it but I still had to replace a box section last year due to the open design of the chassis sections where they meet or have factory holes drilled into them.
Over here in Ireland many mk 7's are being scrapped due to chassis rot.
Good luck with your search.

Wow, thanks for this detailed reply.

I know what I'll be doing this weekend now. I'm keeping my fingers crossed.

[bigjohnthomas]

If it was a leaking windscreen then the insides of windows would be misted over in the mornings
And they don't unless you leave a bag full of wood shavings full of dog piss in the front
Hope this helps
Professor Allcock

[Beaker]

Today I removed my driver's door step cover and peeled up the rubber floor mat and wheel arch cover.

it was clean, dry and with absolutely no sign of water ingress from anywhere.

I had a line of lightly rust bubbled paint under the door step/floor mat join, and some rust bubbles just in front of the seat base. I spent twenty minutes giving it a thorough going over with a wire brush as well as the rust along the front face of the seat base. I managed to Kurust it before it it got dark, so tomorrow I will primer it and re-paint.

I stand by my theory that the rust on the seat base is caused by water from your boots, seeping back off the floor mat.

I was honestly expecting to find a complete mess under there, but it's nothing worth worrying about. I'll check the passenger side tomorrow.

[ned]

Today I removed my driver's door step cover and peeled up the rubber floor mat and wheel arch cover.

it was clean, dry and with absolutely no sign of water ingress from anywhere.

I had a line of lightly rust bubbled paint under the door step/floor mat join, and some rust bubbles just in front of the seat base. I spent twenty minutes giving it a thorough going over with a wire brush as well as the rust along the front face of the seat base. I managed to Kurust it before it it got dark, so tomorrow I will primer it and re-paint.

I stand by my theory that the rust on the seat base is caused by water from your boots, seeping back off the floor mat.

I was honestly expecting to find a complete mess under there, but it's nothing worth worrying about. I'll check the passenger side tomorrow.

Or idiots forgetting to close the window when its raining its not like I have done that on a few rainy nights N