Door-card plastic lining.

If you remove it, your feet will get wet. It's there to mostly stop water running down between the window and door and then into the car, soaking the door card in the process.

No. Water will invariably get into the door since you can't seal a movable window. The membrane is there to prevent that water - or moisture soaking the door card or worse getting into the car.

I am not certain that very much water ever gets down past the door window seal and inside the door, in any car. The window seals on cars seem to be pretty good these days, and the winder and lock motors are not protected. I'm sure it's more to keep out condensation.

It certainly does. BMW E39s are famous for it. The membrane glue fails and you end up with soaking carpets. Re-sealing sorts it. Of course it will vary according to the design of door, etc. But no window seal is perfect.

OK, Thanks to you all - I'll leave it in place!

Huge amounts of water will piss into my Golf without door membranes. The door frames are designed with drainage channels that incorporate the membrane. Granted, the seals are old and have shrunk, but even with good seals, some water will get past.

The key thing is not to start hacking about at it near the bottom. Somewhere near the middle, and you should be OK, as long as you stick some plastic over there. I'd be inclined to use some PU mastic or contact glue to make sure it's good and proper stuck - tape will allow pockets where water can collect and then eventually drip through.

Oops. Had to really - quite difficult to change the riser mechanism without almost totally removing the membrane.

I think I might have a bash at cutting a new membrane from a piece of visqueen or something and sticking it in with contact adhesive.

Cheers.

Not a bad idea. A fairly common belt and braces method with Golf GTI club bods is to make a membrane with a flap stuck halfway down - this hangs inside the door, below the bottom of the membrane proper, between the skin and the frame, the idea being that it makes sure that most of the water doesn't even get a chance to get down to the bottom of the drainage channel.