Connie I edit it for you . To your question I ask some questions . Does the heater work all the time or does it shut off as well ? If the heater works, when the defroster shuts off is it just the fans that quit or is it still running and is the air cold thats blowing out? Have you back flushed the heater core . This could help
https://www.youtube.com/watch?v=dcL_0TWeZJY
you can also back flush with water. hope this helps