Front Heater gives no heat

2001 Windstar with front and back heaters. Plenty of fluid and back is throwing heat. Front suddenly quit giving any heat. Initially after this occurred I was running only front heat. When I turned the back heat on the front heat began working. They both worked for about a week and then the front quit again. I turned the back heat on and got heat there but the front still does not give heat. This is also true with the defrosting. All of the fans work fine.