1998 mercury grand marquis heater doesnt work only cold air comes out the air cond works fine the enginecoolant is new, the engine doesnt overheat.

Either your heater control valve is not properly operating or your heater core is clogged. Locate the heater control valve under the hood, should be close to firewall on heater hose, have someone adjust temperature from hot to cold and see if you can see the heater control valve operating if so replace the heater core, if its not doing anything replace it(heater control valve).