The smell of coolant inside the car is almost always as a result of the heater core (inside car under dash) corroded to the point it leaks coolant. Lack of heat at idle usually indicates a non functioning aux pump.

You didn't say what car what year, but corroded and leaking heater cores are very common in 10 year old cars! My 85 190E heater core has been corroded for years, as is my 90 Volvo! My summer cars, no heat!

Repair will involve removal of the dash board and costly to do!