First make sure you coolant is not low, that would cause that. To test your heater core, when the vehicle is warmed up touch both heater hoses at the fire wall that go to the heater core. Both hoses should feel to be the same temp. If one is a lot cooler than the other one you know that the core...
When it's not starting, test if you are getting voltage to the starter with key power. If not see if you have power to the relay. If no power at the relay you are likely looking at a bad ignition switch (not the key switch)
When it's not starting, test if you are getting voltage to the starter with key power. If not see if you have power to the relay. If no power at the relay you are likely looking at a bad ignition switch (not the key switch)