Ya, something is leaking and causing the coolant to get too low to maintain a low temperature.. If the floor is wet (under your mats ) then it is the heater core your looking at about a 50.00 part.. If the floor is dry then you may want to check your hoses coming out of the radiator to the heater core and back to the radiator. Could possibly be a hole in the radiator but its not very likely.. I would check the floorboard 1st and then check the tubes.. Also check you coolant frequlently (after your engine has cooled down).. Basically the reason why you not getting heat is as the engine gets hot it warms the coolant in the radiator and it flows to the heatercore. When you turn your heat on air is pushed through the heatercore to make it hot. and then it cycles back into the radiator.. Mine had a real big hold in it and I couldn't hold coolant for more than 10 - 15 miles.. So I bypassed it ( I had to drive from KY to South Carolina the next day) I plan on putting a new one in Sunday.