If you haven't already been consulting with a skilled search and recovery organization, I would recommend doing so immediately. While what they can help you with will have it's limits, they may be able to give you some good ideas. I still say drones and drop cameras are your best bets here. Unfortunately, mother nature tends to cover up cars when they go offroad, and unless there are big skid marks, it's just hard to see disturbance to the foliage.
Tulare Sherriff has search and rescue, and there are scores of horse stables, etc. I would have the Tulare Sherriff ask the horse community to join in the search. Horse people are good people, they would help. Horses would be very helpful in this situation; horses and dogs.