on my civic, there's a rubber blanking plug on the passenger very close
to the where the inner fender meets the firewall.  i know your passenger
side is different to mine, but i fixed an identical leak in similar
circumstances a week ago.  crud had accumulated in the crease where the
two panels meet and had built up so that fluid ran over the plug, and
evidently, past an ineffective seal.  i pulled the plug, cleaned
everything up, removed dirt accumulation, and reinserted the plug with a
little silicone grease on it to makes sure it sealed properly.  now the
leak is completely gone.  hope this works for you!

Hi Graham
Had the same problem on a Concerto and as stated it was a build up of
mud inside between plastic and steel up to a hole in that area which
then allowed for water to enter the car. (fun in car washes) Cleaned
out, fitted blanking plug, problem solved, best of luck.

Regards.
Dave.