What is the plastic piece under the front bumper for?
It’s an Air Dam. Course in your case it might be called an air damn!
What is the black plastic under the car called?
On convertibles, it’s a metal skid plate. On coupes, it’s a plastic splash shield.
Is it bad to remove fender liner?
nothing really happens. I’ve had no fenderliners now for at least a year … obviously you get mud and stuff up there, but nothing ‘bad’ happens.