Heat only coming from the dash vents, where is the blend door located?

The vacuum motors are generally long lasting but the lines to them and the seals in the control head do wear out. To test you need to remove the control head. For that you remove the trim panel by removing the screws shown and then releasing the trim clips. Then you remove the screws and pull the control head out and release the vacuum lines. Next use a handheld vacuum pump to apply vacuum to the lines in the connector and track where the air flows through the system. So, you apply vacuum to the orange line, and you should hear the door that changes from interior to exterior air moving, then move to Yellow, Brown and the rest, if you have air movement and can hear the actuators at each line you likely have a bad control head. If you look at the first diagram you can see what the various lines do when the controls are moved.