Or a hole/rust around the windscreen/firewall area caused by a build up of gunk underneath that plastic trim cover between the bonnet and the windscreen.
Also while doing an aircon install I managed to identify a possible weak point in the micra's cabin sealing. If you leave the little fan vent to "fresh air" rather than "recirculate" there is a possibility of water getting through the vent and into the cabin! Also if the seal around the flap which covers up the hole, or the seal around the actual firewall mounting point is damaged or just degraded and broken off the water could find its way very easily into the cabin. These things do deteriorate over time too.
The heatercore is positioned in the middle of the dash, so if the carpet underneath the centre console is damp and the liquid smells really funny you've got a heatercore problem. They're not cheap from Nissan but I see them on ebay for like $350 which is a bargain. Labour is the big thing because you have to remove the dash and the centre structure of the dash/stereo support. This isnt overly hard btw - its just fiddly and you have to be careful and remember which screws go where, etc.
Removing the dash can also help you identify rust issues around the windscreen. Hopefully this is not the case.
Goodluck!