Am having few challenge with my 04 nissan quest AC. The AC only work when I turn it on to highest, and the back vent dose not cool very well.

Sounds like the blower resistor module needs to be replaced. Could also be the switch.

One of the temperature actuators are not working it has 2, one for each side.

Check and see if your cooling fans are working. At least one should come on at all times when the a/c is turned on.

Gear linkage either needs adjustment or has a broken part

here is what you do. look on the passenger side front near the bottom of the radiator. there you will see the dccv or dual coolant control vavle. it will have your two heater hoses running to it and another hose from the radiator under it . once you have identified it , feel and look around it for any signs of leaking. barring any leaks, disconnect the connector, turn the ac/engine on and usining a tester , test for voltage on each of the three wires.if both outer wires have no current ( the middle wire yellow- will have current) then your problem is the control panel. if the control panel is not a navigation it is possible to easily repair it. replacing it can run from 600.00 to 1500.00 depending on who you goe to. oh by the way its still some data that you aint looked at. holla back if you need more help or a repair of the a/c control panel.