Doc: Marty, the idea that I could fall in love at first sight! It's romantic nonsense. There's no scientific rationale for it.
Marty: Come on, Doc. It's not science. You meet the right girl it just hits you. It's like lightning.
Doc: Marty, please don't say that.
When Marty returns to 1985, he goes to his house and gets in his Toyota pick-up. Look closely behind his head when he first gets in the truck, there is no driver-side head rest. But when he picks up Jennifer, when they pull up to the stop sign the head rest is there.
Looking at the three films at a whole, we can note that no fewer than FOUR DeLoreans were simultaneously existing in 1955: (1) the one in which Marty comes in the first movie; (2) the one in which the old Biff comes in the second movie to give the Sports Almanac to his younger self; (3) the one in which Marty and Doc come in the second movie to rescue the Sports Almanac; and (4) the one which Doc hid in a mine in 1885 to be found by Marty in the third movie.