I've got a 2007 V6 4L convert. At idle the defroster or vent heat gets ice cold no mater what I have the temp set at. At higher RPM's it does get hotter and the variable control will vary the temp so I know it's not the dash control. Because it seems to be RPM related I have to assume it's a vacuum related issue. Any help would be appreciated! It's still pretty cold here on the tundra but I took it out of storage hoping that Spring will eventually come.

 

I'm sure there could be many reasons for your heat problems, and I don't want to cause any alarm, but the last time I had no heat or little heat, it was the head gasket. It wasn't my Mustang, but it was a Ford. Rule that out first.