I have just had the same grief (but my alarm was dicky too - but I fixed that now - I think the central locking is still not 100%). I think the only way is to take out the passenger seat and the front piece of carpet at least. You then have to wring out as much water as possible from the foam by bending and squeezing different bits of it - if you really have a lake, lots of water will come out - it is surprising how much water can remain, even if the surface of the foam feels quite dry - either sloosh it down a hole (there is a big grommit up front on passegner side - remeber to remove the one underneath the car as well!) You really have to do a lot of squeezing to get the worst out before any normal drying can begin (otherwise it is like trying to empty a bucket of water with a hair dryer).
Then either leave it running for a day with the heaters on full blast and the foam propped up with vairous bits and pieces so that the air can get underneath (with the window open enough so you can reach in and unlock the door when the central locking fires by iteslf again!) or get an electric heater or mobile air-con in there for a day. I did a day of each with the front carpet in my living room (after wringing!) Once the passenger seat is out, you can take out the front bit of the of the carpet/foam and take it in your house for a few days. You must wring it all out first tho, or it will take yonks. I did wonder about going down the local chemsists, where I am told one can buy silica-gel by the bucket - I might do this as well, but if it is really wet, I think wringing is the only way to get it done (at least if you hope to get it dry before your hair turns grey and the rest of the car is knackered
Also, if it has been damp under there for a while, you may have little spots of rust starting which, being a thorough sort, you will want to treat, so getting under the carpet is the only way I think.
Because of the foam, I think that using the drain holes alone will not help much. Also, the underbelly seems to be double skinned over most of it, so there would need to be two holes I think - not sure, tho.
A similar discussion is going on in 'central locking craziness'
I think I have traced my leak now to the windscreen seal - which is now packed with silicon
I had my cooling system presurised by the local (independant) lemmon-bashers, and they said the matrix was OK. Also, looking at the water I wrung out of the foam, it looked very clean and didn't smell of coolant, so I reckon it was probably rain water. I could feel dampness quite high up the front (behind a secondary plastic/foam cover under the front carpet which I also had to wring out) which is higher than the matrix (I think) so in my case, it pointed to either wheel arch hole (took it to the local jet-wash and sprayed up underneath but seemd to stay dry) or the windscreen. (I have no sunroof)
Sorry that that went on a bit long. I'll shut up now. If I get to the bottom of the central locking problems, I'll post.
Cheers,
Nat
