How Did The Cops Get Louise's License Plate Number?

I just saw the film for the first time tonight.

That part is bugging me at well. They were parked in a huge parking lot and left with no witnesses. Apparently, there are no security cameras. They saw the rapist leave with a woman that no one knows and then he ends up dead.

I can’t see how they would’ve identified her license plate, let a lone connect her to the crime.

Yeah, and the detective also says "The fingerprints on the car [that Harlan's body was next to] match those of your wife." How could the cops have done a match? The only reasons I can think of for someone's fingerprints to be in a file/database are if they've done a crime, or had a job that required security clearance, neither of which is likely to apply to Thelma. There is no national database of fingerprints (or DNA). What usually happens is, fingerprints are lifted, then the cops set about finding (a) suspect/s. *If* they find anyone who seems likely, *then* they take that persons fingerprints, and see if they match up with any of the recovered prints.

(That can sometimes be a heck of a lot of prints. If the crime scene was someplace like a house, there will be prints from every family member, and probably people who visited the house recently. Even if someone lives alone, there's still their prints to deal with, and the perp's prints might overlap with them.)

But getting back to my original point, fingerprints are not some instant magic identifier. It's tricky to collect them intact, and then you have to find the person to match the prints; the prints don't necessarily lead you to the person.

Logically, I think were left to assume they got the license plate number from one of there bf's.

Louise had done time in a penitentiary. That's why she refused to drive through Texas - that's where the penitentiary was located. It doesn't answer the license plate number question but does provide an explanation (maybe) why LE knows who she is.