My heater in my seca sx is not working good, so i changed my thermostat, when we opened it up there wasn't one in there from the previous owner, but still its not working it starts to get abit warm and then it goes cold, and my temp gauge is always next to cold, but where the thermostat is its hot and my radiator is hot to i don't get it plzzzzz someone give me same tips on what might be the problem thank you!