A First Aid Kit should always be carried in your vehicle.
Commercial 'Auto' first aid kits are minimal.  Because of the possible severity & extent of offroad injuries, you should supplement commercial first aid kits. Consult the list below for additional items.     
-  find a sturdy container with organizing pockets or compartments, waterproof container preferred
-  mark the container & compartments clearly (individual items or like items can be placed in individual ziplock bags)
-  find an easily accessible location in your vehicle & secure the first aid kit

                     ! Learn basic CPR and First Aid !
                                                          Remember: there are no paramedics in the boonies!
 

• many of the items that make up a good first aid kit can be found around the home

• 35mm film canisters make good containers for pills, tablets, gel caps, etc.

• items can be found in department stores, drug stores, sporting goods stores, recreation outlets, army surplus stores & medical supply stores

• you'll have determine which items and the quantities required based upon your own first aid skills, offroad experience, and the specific demands of any specific trip

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NOTE: WalMart^® sells a  First Aid Kit (by Total Resources International^®)  with 201 items in a soft-sided multi-compartment organizer hanger bag (~$18) that is pretty well stocked (contents are marked with an  "●" in the following list) I like this one  because it has both carry handles and a carry strap and it is well organized. I carry these (with additional items from the list below) in our vehicles.