Agree with Xulfiqar,
I will try to explain in local lang.
Heater core leakage means. There is a radiator type thing installed inside your car in the dashboard for car heater, The water circulates in this and if its a old car leakage can develope. If ur not using heater u can check this by dissconecting heater pipes from engine to heater core and check.
A/C Evaporator housing well its same like a home window or split a/c water dripping from evaporator in car there is a small pipe under the passenger side dashboard for this water. Normaly when this pipe is blocked water starts dripping inside but in this cold weather i dont think so ur using a/c so i feel this might not be the problem.
Grommets u can search on google images, these are small seals present on ur car floor as well as on small wires or pipes coming from engine area to the cabin side.