Sounds like either a heater hose, heater control valve, heater core, or a combination of these. You might need to remove the glove box and heater fan motor to acess these. I had the same problem last year. Had to replace everything (hoses, valve and core). Once you get the glove box and fan motor out, the rest is not TOO bad to remove, but still a major PITA. Good luck.
BTW, if the heater control valve is shot, you'll find it is next to impossible to find a replacement. I happen to have an extra one that I might be willing to part with...