The obvious question in this case is exactly where the car was found. If it was truly in the desert and a reasonable distance outside Las Cruces (where he lived) one obvious possibility is if the car had broken down then he may have tried to walk for help and succumbed to heatstroke. There have been such cases where the bodies have been found many years later. Of course this possibility depends on the location and whether there appeared to be any problem with the car when it was found.