Hi,
Adapting photon for car is pretty much the same as for a player, you need to update the position and the rotation of the "player", be it a car. The key here too is to only allow user control on the car that owns this instance, all other generated instances must listen to that car synched variables.
Try with a simple sphere first that you control via forces, just to cut the crap of all the various scripts and possible things that needs modifications on a real car behavior.
bye,
Jean