could be sunroof drain but could also be the dreaded rust undeneath the fusebox area.
To get the carpet properly dry you really need to be taking it out anyway. SO remove the front seats, centre console and then get the carpet out. Remove the glovebox and then have a good reach up behind the dash where the glovebox hinge mounts. If your fingers disappear through the bulkhead and rust falls down then you have a problem.
Another item to look at beforehand is the "elephant trunk" under the bonnet. This is the drain for the front scuttle panel and it gets clogged with all sorts of muck over the years, remove it and clean out any crud and re-fit.
