99 F-450 7.3. Heat only in A/C vent. All other settings A/C works but heat is outside air. Truck came from public works in Southern Califorina.

Vent doors not open correct

You have broken vent door on your truck.
Remove the glovebox use flash light to check top of the heaterbox area (top of blower motor housing- toward windshiled area) You will see sponge covered vent.door or you just see outside light)
you can get complete repair kit with necessary electrical actuator about 65/75$ but thing is a job,, basically you have to remove almost whole dashboard to replace them correctly.
good luck.

I the short answer is yes. The truck has a flow-thru vent feature as does most GM products. When the fan switch is off the outside air would flow into the truck.
I thought you could still direct the air by way of the control panel, but by the wiring diagram it looks like everything is disabled when the fan switch is in the off position.

When I turn on the heat in my 2004 mazda mpv the front air vents blow heat but the rear air vents are blowing cold. I have adjusted every setting is there something I am overlooking.

Check the air handler to see if the blower is on. First check the circuit breaker which gives power to the blower.

If the condenser outside is not on, there is no power anywhere. Again, check the circuit breaker.

Also possible faulty thermostat.

make sure coolant is at proper level. if ok change thermostat. if the thermostat sticks it will make hot coolant go through bypass hose at thermostat, and therefore no heat. if that does not help much try bleeding air from coolant system.

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

try checking if the turn off valve needs adjusting stick your head under the hood and get someone to turn the heat setting on and off you can see the valve moving

It is very likely that the Air conditioner evaporator core is restricted due to leaves or some other object may be blocking air flow like a plastic bag, does it do this on both outside air and recirculation setting?

you guys have a vacuum leak some where does the 4wd drive and cruise control work ? both also vacuum controlled that's is where i would start tho