'98 Ford Mustang - Body Part Question
 

What is the lower piece below the door on each side of the car?... it extends in middle lowest portion from left to right

what is the technical name of this trim?... and its NOT the tire fender

Is this what a "running board" is?

side skirt

thanks... know where i can find one for driver's side fairly cheap?

its a red V6 3.8 convertible

the piece below the door is called a rocker panel not always easily replaced
side skirts go on top of the rocker panel usually as an additional piece of trim
a running board is what trucks have, its the little step

Ask DrinkingHard. He used to rock a mustang and a mullet! lol.