If i'm in my truck, my bag goes in the truck.
Maxpedition Riftcore backpack
Inmarsat satellite phone (where I travel, cell service is... lacking)
Mechanix Wear gloves
Wool beanie and/or a desert "sun" hat depending on the time of year.
Lifestraw water bottle/filter
Adventure Medical Kit - Day Tripper (boo boo kit)
SAM splint
Blow-out medical kit with tourniquet
SOL Escape Lite bivy sack
MRE
Petzl headlamp
Spyderco H1 Pacific
toilet paper roll in a ziplock
SOG multi-tool
100' 550 cord
magnesium firestarter and a couple Bic lighters
25' Duct tape
5 Chem-lights
a AA and a AAA LED flashlight
Chris Reeve Green Beret fixed blade
FN 5.7 pistol with 3x 20 round magazines (glorified centerfire 22mag, makes a great small game getter and half-assed SD pistol)
PacSafe 55L stainless mesh backpack protector (to lock the Riftcore to the rear seat frame to prevent smash and grab or rummaging theft if I have to leave the bag unattended for short periods)
There's a well stocked medical kit in my center console with lots of nitrile gloves. I come across car wrecks a lot (usually one or two serious wrecks a month) and there's never enough gloves to go around.
During the winter I also throw a Hill People Gear Mountain Serape in a stuff sack on the back seat.
Pelican cooler locked up in the back of the truck with a minimum of a case or two of iced down water.