No heat on 1993 Ford Ranger

I put in a new thermostat, but the heater still only blows cold air.

2 answers 1 comment
HEATER CORE MAY BE PLUGGED. GREG
How would I go about unplugging it.
Is there any valve connected to the core?
grab the heater hoses at the firewall and if one feels cooler then the other just flush it out with a garden hose,reconnect the hoses then check for heat.i would buy a $4 prestone hydrometer to check for boil over and frezze over protection also.