Your problem could be one of a few different possibilites. A low coolant level could cause the heater to blow cold even if the engine is running at normal temp. This vehicle does not use a heater control valve to stop coolant flow through the heater core; rather it uses a blend door and plenum chamber to mix hot and cold air from the AC evaporator and the heater core. You may have a problem with this blend door. The plastic door can actually break and leave the door in the full hot or full cold position. This is an expensive repair as the entire dash, instrument panel and HVAC case must be removed to access and replace this door. If this is in fact your problem, it is a wise to replace the heater core at the same time.
Check your fuses. I drove without heat for 4 years. Went to replace the blend door actuator and the old one worked after we tore apart the dash. I had a blown fuse. Go through your owners manual because there are several fuses that tie into the heating system. I had already had the radiator replaced, the heater core flushed and had several certified mechanics look at it. I have a 1999 Expedition
