If the person driving the vehicle had a cell phone that would be an easy thing for them to link. And they said over 70 subpoenas have been issued, I believe several would be for surveillance video from businesses. In those they may spot the pickup.
Here is an excerpt on tower dumps and how the FBI uses them. Maybe they did something similar if they could map where Libby's phone pinged then search the numbers to find a phone that pinged the exact places and times.
"Tower dumps aren't like going after targeted cell phone data on a known suspect; they are more like casting a limited dragnet, pulling in the phone numbers and (rough) location of everyone in the vicinity of the event. And tower dumps are usually obtained without a warrant, instead utilizing a "court order" with judicial oversight but a lower burden than probable cause. The FBI actually received more than 150,000 registered cell phone numbers from this particular set of tower dumps, despite picking the most rural locations possible. What the case agents wanted to do was scan the logs from all four sites on the belief that no single person was likely to be at all four banks during the robberyexcept for the robber.
So the FBI dumped all the numbers into a Microsoft Access database and ran a query. As expected, only a single number came back, the robber."