I have been getting a coolant smell through the vents when the heater is turned on in my 98 ML320. I haven't had to add much water to the coolant system in the three and a half years we have had it, maybe a two or three pints total in the whole time. I'm assuming I'm starting to get a leak in the heater core. Has anybody here ever tackled the heater core? How hard is it to get to?