My understanding was that they identified the cell that had the last known contact with their cell phone and used that location for the start of the search area.

A friend at work used to work for Motorola, and he said that the only way to get a ping from their cell phone would have been to fly a helicopter below the ridgeline to avoid other cell phone traffic at a distance, and then to listen for the individual ping of the cell phone with a higly directional antenna. Once detected, they'd need to get the ping in at least two other locations in an attempt to triangulate the location of the source (the car).