Please see my final action on this forum with images. BMW: E39 rear n/s footwell wet.... : BMW 5 series Forum • BMWLand
Alternatively here is the text.
This is my conclusive action to prevent the leak in the rear footwell before refitting the carpet, centre console and seats, followed by a prayer that it works. :roll:
I have cleaned both cabin filter housings and sealed the edges with Butyl adhesive; this does not set so will allow removal of housing even though it will still be sticky.
The water is coming from the drain of the cabin filter housing and trickles down, entering the subframe-sill-rear footwell via small ripples in a seam weld at the rear of the driver's side wheelarch. These are intentional shapes created by BMW, not damage or deterioration!
I do not know the purpose of these holes/ripples, only that they are causing a flooded footwell so have sealed them with more butyl adhesive. I have also sealed the hole below these that supports the washer fluid bottle. I soaked the windscreen and roof with a hose and only a very small drip appeared in the car. :?
Next I heated the seam weld in the car to remove the remaining water
and then promptly sealed the seam with more butyl.
Tested again with a hose, before protecting the sealant from the carpet foam with aluminium foil.
The car is back together now. The car has been parked on a slope with the rear R/H being the lowest point as the battery has been removed to allow safe removal of the seats and door panels. I expect a similar leak to appear on the L/H side should the car be parked on a L/H slope. In the near future I will remove the L/H side wheelarch and search for similar holes that I will also seal-up. I will inspect carpets in a fortnight and hope that everything worked............
Any comments welcome.