How about taking prints from the license plate that was found on her car and prints from both cars.
Sadly ironic, since she's probably fertilizer by now.
This blog has various update stories on the right hand side:
http://www.missingabducted.com/
This Times dispatch article says they are still looking at Fawley for this:
"law-enforcement source familiar with the case said the plate may have been in the possession of Ben Fawley, a 38-year-old amateur photographer and acquaintance of Behl's who was one of the last people to see her."
http://www.timesdispatch.com/servlet/Satellite?pagename=RTD/MGArticle/RTD_BasicArticle&c=MGArticle&cid=1031785347331
Also FYI-
A concert featuring some of Behl's friends and favorite musicians -- Shane Hines, The Brindley Brothers and The Echoes Band -- will be held in Vienna at Jammin' Java on Sunday at 1 p.m.
Proceeds will go toward the Friends of Taylor Behl reward fund, which currently stands at $14,000, said family attorney George Peterson.
VCU has also posted a $20,000 reward for information leading to Behl's whereabouts.
It takes a real genius to steal a vanity plate such as that and actually try to put it on a vehicle. Just a hunch but it was probably stolen from the vicinity of the second "person of interest" (younger guy) but I doubt that it's connected. Some treehugger teenager probably wanted to put it on their bedroom wall.