I was 20 and home on spring break. I am driving home around midnight from my (then) girlfriends house. As I crested a small hill a car coming the other way illuminated my interior.
I look towards the right to avoid the glare and I see through the rear view mirror that there is a person in the back seat of my car.
Behind me in the passenger side back seat is a person in a plaid green/brown shirt with one of those extra wide collars from the 70s. I can make out the persons chin and nose and that shirt but everything else is just an outline of shadow because of the way the light is hitting my front seat with me in it.
I freaked out and slammed on the brakes. I look back and there is nothing behind me. I put the dome light on and kept in on the whole ride home.
That Sunday, after church, my Mom and Dad took me to breakfast and I tell them the story. I thought it was a funny 'man I must have been tired, but that scared me good' type of story.
Except my dad stops eating, and I remember this specifically, he deliberately put down his silverware and in that "being careful what to say" voice asked me to describe the shirt again. He says that was exactly what his dad's work shirt looked like and got upset with me. He thought I was making it up to mess with him.
We kind of let it go after that, but a few years later my dad got some old slides his sister was given during my grandfathers estate/will stuff, and called me to come visit to take a look at one of them.
It was a picture of his dad in the shirt. I never met my grandfather, he died long before I was born, and most of the time I know I must have seen that slide at my aunts as a kid or something (even though I have no memory of ever looking at any slides with them).
My first thought at the time was that it was just me in the rear view mirror with some trick of the light, but I was wearing a blue hooded sweat shirt.
It was probably just my brain losing a connection, but that doesn't stop me from not wanting to check my review mirror when a car passes going the other direction at night. I still put the dome light on sometimes.