If you smell it, either the fresh air intake for the cabin is picking it up from a leak (cap or hoses prime suspects) OR heater core/fittings loose or leaking. Given that kind of volume, you'd notice heater related stuff pretty quick I'd think. I don't if they're much in use any longer, but I used to test coolant systems with a hand pump that screwed onto the neck in place of the radiator cap. It would pretty quickly spot leaks that don't happen when it's cold, and it's not always practical to find when the motor is hot.
2003 RAM 1500 Quad Cab Patriot Blue/Silver