2002 Blazer S-10 heat stays on
 

I have a 2002 Blazer S-10. The control knobs work good controling both ac and heat when on. But when everything is turned off there is still heat coming out the floor vents when I do 40mph or faster. Could it possible be that the blend door actuator is not working properly? if so does any one know where it is located.
Thank you for any advice