I tore up my carpet in the back of my '98 190P. It was fuh-gly.
I can confirm it's 1/2" plywood under the carpet. Glued to the steel floor of the van. If you try to screw down the board to stop the squeak, you would have to go through the sheet metal. And that might bring on rust and water ingress.
I pulled all the tacks and scraped the carpet glue off the top surface and put down black foam tiles that you get for exercise equipment. It's much softer than the carpet and warmer in the winter, and the drawers still clear it. I put even thicker and warmer foam tiles in the tub itself.
We can walk barefoot in -8 C and not be cold.
That's NOT stain on the plywood. Soy sauce had soaked through the carpet
. But you can see the steel floor just below.