Year around it's important to keep jumper cables, a basic tool set, some extra engine fluids and a working flashlight in your car. There are also some seasonal items to keep in the trunk which include warm clothes in the Winter and cool clothes in the Summer. And going even beyond that, it's important to keep a couple days worth of food and water, and a good first aid kit in your vehicle.
Another thing to keep in mind is your spare tire and jack. Checking your spare tire air pressure needs to be as regular as you check your car tires pressure. It needs to be kept at pressure in case you ever need to use it. The stock jack and lug wrench are decent at best, so if your looking for something better than the stock one, a hydraulic jack and heavy duty lug wrench can be purchased at any store for pretty cheap.
Above is a picture of my trunk with the emergency items that I keep in it. Most people don't use their trunk for much other than collecting junk, and even with the preps in mine there is still room to put a few other things in there if I needed too.
To keep things organized and from flying around the trunk while driving I use a UTG Duffle bag to keep my extra clothes and smaller things in.
Below is a cart of the things inside the duffel, with thumbnail photos underneath of the contents. Click on the thumbnails to open the full size picture in a new window.
Outside of the duffel bag I also keep a small tool box with tools in it, a small emergency kit, a large 206pc tool set, and some engine fluids and in a plastic crate do they don't roll around in the trunk. Below is the cart and thumbnails for all the trunk items.
So there you have it, a pretty complete car emergency kit. There could be a few things I left out, but prepping is an ongoing activity. Outside of the trunk I keep a small hand powered LED flashlight and my Leatherman Wave multi-tool in the cab with me. I try to get things when they're on sale, so if you see a good deal snatch it up, and add it in. Car emergency kits are important because they could get you out of a pickle, but you could also use your preps to help someone else who's broken down or needs a jump at the grocery store. Part of being a prepper is using our knowledge and intuition to help others who are in need.
Also, here's a picture of my trunk with all my emergency preps and my Get Home Bag in it to show that it all fits with room to spare.