OK first off i will say sorry if this is posted somewhere else.
I have a 98 GMC Envoy, over the summer I changed the radiator, it over heated so I took the thermostat out and it ran fine, as it got colder out I had heat but not much ( because the thermostat was out), but before I had time to out one in I completely lost heat then a day later it over heated,

So I changed the water pump, put a new thermostat in and still no heat.I then went and had the system flushed just in case there was a blockage in the heater core.After that there was still no heat so I was told the actuater for the blend door was bad and was behind the glove box but it wasent what the guy showed me

( the above pic is the actuatuar he showed me )
But what I found was this


When I reached up in there I closed the door with my hands nd it made the heat a little warmer but not enough to make a difference when it -10 out.please any help would be awesome.

 

After you have run the vehicle for awhile; (enough to open the thermostat) are both the upper and lower heater hoses hot?
If not, you either have blockage in the core or air in the system .. Try burping your cooling system ( do a search if you don't know how ). If that doesn't work and you know the thermostat you installed is good, replace the core.

 

heater core is fine, both lines hot, thermostat opens,its something else

 

What your mechanic showed you is behind the vacuum actuator that you found. Remove the vacuum actuator and you should see the motor-driven actuator.

 

I will look thank you, what door is the vacuum actuator opening and closing then?

 

Couple of diagrams might help: