Yes sand the plastic first.
Sanding textured or grained plastic will ruin it. Clean it well, you can use scuff stuff and a grey scotch brite very lightly but you need to be careful.
Follow the directions as AP have kind of fallen out of favor as the regular paints get more flexible.
Ad-Pro is for adhesion not flexibility.
For those Mustang panels they should be bare plastic that is dyed/colored. They were not painted. Clean the panels well. Like I said above a grey scotch brite and scuff stuff used very lightly
works well. Or Dawn and a grey scotch brite. Then use SPI 700 W&G remover and wipe everything down like you would an exterior panel you were going to paint. Use a dedicated paint prep wipe, never use microfiber cloths.
Apply the Ad-Pro per instructions and proceed with your color coat. Read the instructions in the Tech Manual as well.