Yes people have paid me to organize their cars. The car is the extension of your home and office. Many of us spend up to two plus hours a day in the car commuting to and from work. Others spend all day in and out of their car in a sales territory (this used to be my life).
What should every one have in their car? Lets start with the glove compartment. We all should have a car manual, a flashlight, and insurance card. I also have a flare, and a light that charges through the battery charger(a gift from the Dad). The glove compartment also holds my ever important coupons. I keep all my Jo Ann Fabrics and Bed Bath and Beyond coupons in my car. My garmin (my most precious possession) also lives here.
The only thing I keep inside my car is my umbrella. At different times you will see the book that I am reading on the front seat. I tend to pull over and read in between organizing jobs.
Last but not least, the trunk space. The trunk contains my battery charger cables and a shovel(which comes in handy during Chicago winters). I also keep my reusable bags in the trunk(that way I always have them with me). Depending on the jobs I am working on, I might keep some supplies in my trunk as well.
In my former life as a sales executive, I kept two file boxes in my car trunk. One of them had client files and the other one has work brochures. I used my passenger side seat as a desk. I kept my files for the day on it, along with a note pad to take notes from client phone calls.
So no matter what position you have in your life, remember that your car is an extension of it and needs the same organization as your home and office.