The heater in my 2006 Pontiac vibe only blow warm air when car in motion Why?

SOUNDS LIKE A CLOGGED HEATER CORE ,THE CORE IS A MINI RADIATOR INSIDE CAR THAT COOLANT FLOWS THROUGH,IT WILL CLOGGE UP HERE AT STACKS AUTO SOMETIMES WE CAN CLEAR THE CLOGGE OUT AND GET HEAT AGAIN .GIVE US A CALL WE CAN HELP YOU 631-422-4010 STACKS AUTO....OR STACKS IN WEST BABYLON 631-539-0930