So is it in the radio under the drivers seat or just the footwell. Usually it is the windscreen or the door seal that lets the water in. Get some insulation tape & seal the windscreen as a temporary measure. Check to see if the door seal is seated correctly. If its in the radio well then I would say door seal.
Did some investigation and I couldn't get the mirror off, but I got the carpet undone, flooded the car several times with a watering can and bucket and it was bloody bone dry around the area where its flooded.
Seems like wherever the leak was, it hasn't returned. Either way, my radio wire has corroded and come off again in exactly the same fashion as the diesel espace.
Brian, also, yes, it was in the footwell, there was soo much water, it was about 2 inches deep in old money. Certainly visible anyway. I've not taken the front seat out to see if the radio is drowned as well, but when I opened the door to see, nothing else was wet but the footwell and the cubby hole.