Filthy carpet trim - how to clean?

[HPsauce]

Following up from my flood the other day I started looking at the state of the footwells and carpets, it's sandy beige (papyrus?) trim. The drivers side is really bad apart from the overmat which I bought new a while back.

The main foam-backed carpet is badly stained, highlighted by the recent soaking. As are the fixed carpet sections around the pedal area.
The transmission tunnel side panel I cleaned once before (last time I did the drains!) as that was stained, I thought it looked like brown shoe polish from the location and colour.

The main carpet I've now washed in the shower using a scrubbing brush and washing machine liquid detergent. I did it 3 times as a seemingly endless amount of dark brown muck came out. It did seem to be water-soluble (especially with detergent) with only a small amount of gritty residue.
It could ideally be done few more times, but I haven't the willpower and it's looking so much better now, draining out hanging vertically before I put it somewhere to dry.

But I am mystified what could have stained it, the amount seemed ridiculous for just shoe polish. The dark brown colouring was in the carpet side fibres and backing material, relatively little came out of the foam padding on the other side. Though I do wonder if it came out of the heavy black soundproofing material permanently fixed on the transmission tunnel as that was soaked too.

That leaves the transmission tunnel trim panel to do again (a bit of black shoe polish from me) and the footrest (both come out when I attack the drivers side drain the drain) plus the fuse panel cover and the carpet under the pedals.

The last one, which also goes round the kickdown switch, looks tricky to remove. Does anyone know how?