Year around in my truck I carry:

Umbrella
Flashlight
Batteries
Watch cap
Gloves
Becker Brute
Jumper Cables
Flares
NRA kit (Blastmatch, Whistle, Mirror, Tinder)
AMK Heatsheet
True Nord compass
Diamond stone
Coat
Tools
Tire repair kit
Plastic water bag
Spare boots
Block and tackle
Large automotive mirror
Large HD leaf bag
Spare serpentine belt
80' snare wire w/ locks
Fishing gear
Spare change of clothes
Campsaw
Axe
Coffee pot
Collapsible dog bowls
First aid kit
8X10 tarp
Can dogfood (4)
Water (2 gal.)
Can stew (2)
MRE"S (2)
Can vegetables (4)
Coffee (1 lb)
Coleman 1 burner whitegas stove
Heatdrum
Coffee cup
Mess kit
Utensils
Large soft-side cooler
Shovel
Sleeping bags (2)
Closed cell foam pad
1 pt water bottle
Sterno (4)
Lensatic Compass
Bic lighters (2)
Treated strike-anywhere matches
Chemical handwarmer
Chapstick
Emergency candles (4)
Sunscreen
Paracord
Boullion cubes (6)
Potable Aqua
8 X 25 Binoculars
Empty 3# coffee can
Triox bar
Surgical tubing (4')
Mora knife
N95 Masks (2)

Depending on the weather and my destination I also might throw in tire chains, tighteners, and a floor jack.