How do you flush out a heater core on a 1994 Plymouth Acclaim 4 cylinder? PLEASE HELP!?

The car doesn't have heat so I put in a new thermostat and that seemed to work for like a day then it went back to not blowing out hot air just barley luke warm. So maybe it might be the heater core is clogged. Anyone have any ideas what the problem could be? Please help if you can. Its starting to get very cold in Ohio. Is there another name for a heater control valve for my car? I cant seem to find anything for that anywhere