The key thing to remember about the car is that it had no front plate in the videos, so it stood out from Elantras registered in WA and ID. But in a college town, there might have been dozens of Elantra's with no front plate. I notice they didn't point that out at all. They just kept asking about Elantras until his finally rose to the top. They probably didn't want to tip anyone off so that they re-registered their car.
We may be surprised how many matching cars they ran down and eliminated one way or another. Then they got to his and couldn't eliminate him. And one thing after another, the pieces started falling into place. But they apparently couldn't get his DNA. Without a match to that DNA they had, they were stuck. I think that's when they took some of his parents' trash back in PA and proved the familial match. I could be very wrong, but I think that was done before he left school to drive back home with Dad.
Working backward on DNA is an investigative nightmare. In my hometown, a young girl scout disappeared and was murdered in 1975. A semen sample was found on her body. They had one suspect, a young man from her neighborhood, but the case went cold. Forensics were not good in 1975 but miraculously, that semen sample was preserved properly. 20+ years later, still working the case, the police asked everyone they could find of a certain age range that lived in the area of her home for a DNA sample to try and find or eliminate suspects. It didn't give them a suspect, but it did give them a long list of people NOT involved, including that young man who spent 20 years under a cloud of suspicion. Several years after the DNA sweep, they arrested a transient for a different rape and murder, and when they ran his DNA looking for other possible crimes, his DNA matched the sample from her case. When confronted with the evidence, he confessed. Because all they had was the DNA, finding him at all was pure dumb luck.