2000 blazer heat issues

I was reading about other heat issues here but mine is a little different. When i turn the heat on the vents or the floor i have heat but my defrosters are cold. I was low on antifreeze so i added that and changed the therm. I also replaces the radiator cap. Once that all didnt work i took the cap off and turned it on thinking i had some air but none. Any other ideas?

you might want to check the thermostat, I replaced mine when I was not getting any heat ... worked after I replaced it.

He said he replaced the thermostat and also said he has heat everywhere but the defroster vents, so he doesn't have a heat problem.

It sounds like the blend door behind the dash is stuck.