Welcome to the forum dude. Most probably is the heater core, but have a look see just to be sure. If it is the core, sounds like a big job, dash out and everything. If your after a quick fix, either get some of that crap you put in the coolant to block of minor leaks such as chemiweld, downside, should only be temporary fix, may or may not work, might bugger up something else, probably not recommended. Or just bypass the core by sticking a pipe between heater hoses at the firewall, downside, unroadworthy and no heater.