I will mention a few critical items to carry in your vehicle in a minute but first ask yourself an even more important question: do you check out your tire pressure, radiator level, and fuel amount before you leave each time on a trip into the wilderness? Most professional mechanics I interviewed while doing research on roadside survival strategies, indicated that motorists get into trouble on the backroads by not taking time to check on those key areas above.
Once you have done this, what needs to go into your vehicle should cover the following five areas:
1. Shelter Carry a sleeping bag or a wool blanket. I once had to weather out an entire day in my truck during a blizzard on the highway, which was shut down due to an accident. A wool hat, gloves, extra jacket, and spare boots should also be a part of your kit. Stow it all in a duffel bag and throw it in the trunk for an emergency.
2. Water Because I live in the arid Southwest, I carry 20 gallons in my truck. You may need less but plan on having roughly two gallons per person per day. Water is so precious-you can't overdo this one.
3. Food Yes, you can live for days without it, but why suffer? I carry three or four MRE's (Meals Read To Eat), and at a minimum, a jar of peanut butter and crackers. Keep in mind that you need food that won't freeze solid or else you'll be visiting the dentist after your rescue. For a weeklong trip, I would pack a cooler full of my favorite pre-cooked dishes in Tupperware. Meals like rice and bean burritos, chicken stir-fry, and pasta are my usual standbys along with fruit and veggies. These can be eaten cold or heated on a camp stove.
4. Signaling Will your cell phone work when your jeep is stuck in knee-deep sand at the bottom of a canyon? If you travel extensively in remote areas, I would highly recommend a PLB (Personal Location Beacon) like the Microfix PLB 406 MHz GPS Personal Locator Beacon made by ACR. Short of that, strip off the vehicle's mirrors and use them for flashing a plane, use the rubber mats on the flooring to create a smoky signal fire (if it's safe to do so in your given environment), and periodically lay on your car horn in patterns of three, which is the universal distress signal. If your new SUV failed you then you have no loyalty to it.
Lastly, consider buying a can of Fix-A-Flat and a mini air-compressor (these cost around $20) in case you have to contend with a flat tire down a windswept road in the wild places.