Don't have a trunk. But under the backseat live the jumper cables, some ratcheting tie downs, a wool blanket, a gallon of potable water, and my scissor jack. The spare gloves, socks, first aid kit, flashlight, and peanut butter crackers live in the second glove box. It's not overprepared. It actually disgusts me how few people have cables anymore.
We call it a boot here. Antifreeze, car kit, bags for life, and twelve bags of compost that i was too tired to take out and stack in the garden this afternoon.