Ian. If you look up above the steering column from inside the footwell is there a bunch of cables that come through the middle of the horizontal surface? On the Plus 4 that is where the main cables enter the area behind the dash (on the right hand side, so passenger side on mine). Naturally the Roadster could be completely different.
It's a completely different layout. If you look at the first picture in my post about the cover, you'll see a strangely shaped box on the horizontal surface you referred to. Inside that are the fuses & engine management bits. There's a large hole in the bottom opening to the area behind the dash. Most cables go that way, but a lot go through the grommet you can see just below that box. That's where most of the water goes in on my car.
Ian