its not the easiest job to do yourself but condensation on windshield around passenger side with coolant smell is a good hint that your heater core is going on the fritz. as for the thermostat its two 10mm nuts that hold it in place under the distributer, less than a 5 minute job to change the thermostat. yes, some day your car could overheat from the lack of coolant being leaked out of the heater core. look at your carpet around the passanger and driver floor and see if there is any dampness. you do need to replace it if the heater core is bad, i would monitor coolant level, and the coolant smell along with condensation on the window for a week before i decide to go change the heater core. keep us updated. and i sent you a link in your pm to download the FSM for the corolla