Heater controls, 2000 S-10

How can I troubleshoot the vacumm controls of the heater/ac? turn on the heater it comes out the defroster, changed the vacumm control unit

Is this old-fashioned knobs and levers or an electronic control? Did you change the control in the dash or a vacuum motor on in the ducts? Do you have vacuum to the control? If there is no vacuum, these systems default to defrost flow so you can see where you are going (even if your feet are freezing).

Start at the source, check the vacuum accumulator and check valve.

And the vacuum lines.

everything works except for the heater. I have checked all vacuum motors, and I have vacuum at the control. Could the diverter arm inside the duct be broken?

Do a web search on Heater Treater - it’s a fix for broken diverter doors.