My daily driver, a 97 jeep grand cherokee, stays PACKED full of gear. Because I live in a cold snowy place, it is mainly prepped for getting stuck in the winter. I also have it set up so I can live out of it if I have to. Here's a list of what I have,but it probably won't cover everything.
Full array of automotive fluids- oil, transmission, radiator, brake, power steering, and windshield wash.
Tools: ratchet and socket set, 4 way lug nut wrench, a bottle jack, tire repair kit, 12 V air compressor
Standard fully stocked 72+ hour bugout bag
Small shovel
Axe
Several gallons of water
A sleeping bag that lives in the back seat
Piles of empty junk food wrappers
A tent
Come-along and a tow strap
12 V spotlight
Chain
Ratchet straps
WD-40
Haynes vehicle repair manual
Electronics repair box (pliers, wire strippers, spare fuses, tester, terminals and connections, etc)
My girlfriend and I basically lived in it over the last winter (small town, nowhere to go if you don't party or do drugs except for maybe the library) and it was quite comfortable with all that stuff.
A wadded up sleeping bag or pile of blankets makes a back seat FAR more comfortable to lay in.
If you keep food in your vehicle, check for mice every once in a while! I found a mouse nest under my seat during the winter, but I didn't find any other sign. I think the cold killed them off before they could do much.