Air/heat not switching to defrost

Hello and welcome to 2CarPros.

When you switch to floor does it switch? I ask because your vehicle uses a mode door actuator and the re-circulation door actuator both when switched to defrost. The
mode door is actually not hard to get to to test or replace. It is item 10 in the first image and the part in the second image with the two screws.
To get to it you remove the trim panel under the dash on the drivers side which is the hardest part to remove as most of the trim on the drivers side dash need to be removed as well. That is the item in the third image. Most of the trim that has to be removed snaps into place with clips and plastic tabs. You use a plastic trim tool or stiff plastic tool to pop them loose.
First is the end cap on the instrument panel (img4) Then the headlight switch trim (img5)
Next you remove the left side glove box, it tips down, then you reach in and press the two side tabs in to release it and tip it down and out.(Img6) Next comes the trim panel between the steering column and the dash, it has a single clip at the top and tabs on the side(img7)
Next you remove the steering column trim covers, There is a screw in the bottom one and the top one clips in place(img8) Now you remove the front left side floor console extension, two clips hold it(img9) and the right side extension as well, it's a mirror image of the left side. And then you can remove the ashtray assembly. It clips in place on each side of the console.(Img10). Now the trim panel itself can be removed (img3) and you get to the air duct. Like all the other plastic it snaps into place and has a screw holding it. Remove it and the actuator is about halfway up the side of the HVAC case.
With the panel off it is easy to watch the actuator and see if it moves. A common failure item on these is a small plastic gear inside the actuator that moves the main cam. They split and the motor moves but the cam doesn't. It's an AC Delco #13267333 and is cheaper than buying the complete unit (Gear is around 5 dollars the complete actuator is around 40). To replace it you must disconnect the battery negative cable, this is very important as the system will calibrate as soon as battery power is reconnected. Then the actuator screws get removed, and you pull the actuator out. Release the clips on the actuator housing and split it open. Remove the damaged gear if it's broken and replace it. If you replace the complete actuator itself disconnect the electrical connector and then reverse the procedure to install it.

Now that you have read all of that, here is something to try before all this. Go to the instrument panel fuse box and remove fuse number 17DA. It powers the HVAC logic controller. Keep it out for about a minute. Plug it back in and try the mode controls and see if they now work. Sometimes this reboot will reset the controller and it will reset the actuators. If it works, great, if not follow the steps above to visually inspect or replace the failed actuator.

If the actuator is moving but the air is still not switching to the defrost then it could be internal to the HVAC case, however in many cases you can test this by selecting defrost, then watch the actuator. Once it stops moving you can remove it and you will see the shaft it turns, take a pair of pliers or similar and try moving that shaft and see if the airflow changes. If it does then there is an issue in the actuator. If moving the shaft does nothing it is internal to the HVAC box.

Is your car the same as in the post 2012 Regal with the 2.0? If so that looks like the end of one of the mode door levers. They are up at the top left of the HVAC case and behind and to the right of the instrument cluster. To access them you pull the cluster out and you have access. Did you find that on the drivers side floor?
The images are what the service info shows.