Might be something obvious!! I had the same issue a couple of weeks ago. scratched my head for five minutes bumped it for ten looking below dash then realised my floor mat had a tiny hole in it. The snow had been melting off my feet and seeping underneath!!!
I'll guess its the pipe to the rear washer that runs down there. It may have leaked a bit when you tried to use the washers & they were frozen, unless its very wet & has popped off? Does rear washer work ok?