03 IMPALA.It won't switch from defrost to vents or to floor,also where is actuator?

some of the dash may need removal to fix that
not a normal service or maintenance job

faulty directional flaps /or switch levers ?

directional flaps either motorized or vacuum actuators

an example 2003 will be close to this set up
How To Replace Chevy Heat Door Actuator Impala 2006 2013

Mode door or temp. door actuator, plastic gear is worn out . Can you change air flow , defroster , vents , floor direction etc... ? Change heat to A/C etc.. ?

I'd have to check actuator movement for defrost. The actuator is electric according to my database. The actuator should be under dash at end of hvac housing.

Blend door actuator is usual culprit here

On a 1996 model you probably have vac actuators that control air flow. Look under the dash and you may be able to see the actuator that controls flow to the defrost and vents. There would be another one for temp control.

Check the power or vacuum to the actuator switch for the blower door. If you have these then the actuator motor is probably bad.

Most common cause is a bad blend door actuator. This controls how the air is diverted, Sometimes the actuator will become stuck in one position. There are 2 types, a cable driven or electronic driven. Search for blockages underneath the dash that could prevent the actuator from moving. Also, it's possible (if it's cable driven) that the cable is either broken or loose to the point where it won't move the door any longer.

The heater blend door actuator (motor) is faulty.Replace actuator

first check to see if you have power going to the actuators when you switch the switch to it location if no power replace controller if you have power and no movement from actuators replace actuator. if actuators move remove hvac box and check for broken blend doors.

you have to replace a vent actuater