Check heater valve first
Before you decide to go about the business of tearing your whole dash apart or paying someone a large sum of money to do the same, check whether or not it is your heater valve that needs replaced.
I thought my heater core went bad a couple of weeks ago and was planning on replacing it too. I was continually filling my radiator on my 190e until the problem became so bad that I could not drive the car. It was that day, with steam pouring out from under the hood that I was able to spot the problem. The leak was coming from just inside the fire wall.
The heater valve is a small vaccum driven valve located inside the firewall that connects to the hose that leads to the heater core. Apparently, the shaft that turns this valve broke, allowing coolant to leak from the unit.
The heater valve is fairly easy to replace and very inexpensive ($15.00-$25.00) If you are lucky, this may be the only thing you need to do in order to restore the heat inside your car!!!!