Hi Daddydaycare,
I've just gone through a lot of trouble with a similar scenario in my '55 reg D5. It seems XC90's leak more than Land Rover Discoveries.
I suspect my situation was not identical to yours, but my story might help you in some way. I intend to do a full post on this when I've finished the other stack of work on the car.
My car also has no sunroof, so no drain tubes to worry about but it still had bad leaks on both sides, passenger's and driver's, and I had literally litres of water sloshing under the carpets on both sides, front to back. Mine was a long-standing leak which rusted the parking brake to destruction and the anti-yaw sensor as well.
Youtube yielded a few helpful videos. Under the plastic windscreen scuttle panel is the metal body of the car underneath the windscreen wiper motor and linkages. At either end of the metal panel are the plastic drains for that area. You say yours are not blocked, which is good.
However, according to the Youtube videos, the plastic surrounds around the large apertures in the metal scuttle panel which allow air to pass into the ventilation fan (UK model passenger side) and the wiring loom to the ECM (UK model driver's side,) may not seal properly. I surmise that if, in heavy rain or if the scuttle drains are blocked, water may back up, seep under the plastic surrounds and over the metal lip around the aperture, and get into the car that way. It may run down the wiring loom, over the ECM and towards the fusebox which is roughly underneath the ECM, or at least close to it.
So I removed my surrounds on both ends of the scuttle (passenger's side clips over the metal lip, driver's side is screwed down), thoroughly cleaned the plastic and metal faces and sealed the joins with clear silicone before reinstalling the plastic surrounds. Then I ran a bead of silicone around the joint again. This may help, but sadly it only solved my passenger's side leak.
I still had to remove the carpets, which entails undoing the front and rear seats and removed the amplifier and corroded anti-yaw sensor under the driver's seat, and DVD satnav under the passenger's seat. I discovered the driver's side leak to be coming from an unsealed seam between two metal panels; one forming the driver's footwell from below, the other the bulkhead/ firewall behind the engine from above. They meet and overlap in line with the steering column where it passes through to the engine bay. For some reason I can't attach a photo today but there was a hole between these two panels to the right of the steering column gaiter. It's unlikely yours is leaking here as the fusebox is too far away, but it may be possible that a panel junction seam in your car is leaking closer to the fusebox? I had to seal mine with Grey Stripe seam sealer from Halfords, both inside the car and within the engine bay. It's a pig of a job to access the seam in the engine bay but it has solved the problem. I've tested it with a hose and we've also had torrential rain since then and there was no water in the car.
I hope this is of some help to you.
Please let us know how you get on.
Aidan
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