Ok, so I was getting something out of the back seat whilst simultaneously filling out a withdrawal slip in the bank drive-thru. I had the pen in my hand, reached into the back seat and ended up with a 2-inch long ink mark on the headliner. Anyone have any advice on getting it out?

my only suggestion? lick your finger and rub it! but if i had to guess, you may have tried that

What is your headliner made of?

go to a auto parts store and get some upholstery shampoo it will take it out.

tide pen?

303 Carpet & Upholstery Cleaner

Paint thinner. I'm assuming the headliner is not black (which could make the area you clean lighter if it was black, but you won't notice it if it's tan/gray)? Anyways, all you have to do is get a towel, dip a small corner in some paint thinner (don't get a ton of paint thinner on it), and gently rub at the spot with the area of the towel with paint thinner on it. I don't know if I ever had to do this for a pen mark, but I did do this at work for other stuff on headliners, door sills, doors, seats, etc.

Don't rub too hard or you'll get a lighter patch in your headliner, and you can't fix that.

Nail polish remover then blot with clean dry towel. Gets out pen every time, though it might take a few applications to completely remove it. Be sure to blot not wipe, it will smear if you wipe it.