In this scenario the FIRST thing one should do is secure the scene. Look for other safety issues like the downed power lines like Roarmeister mentioned. Get flares out, block the scene with your vehicle far back as a buffer! (Leave a path for emergency vehicles to enter and exit) Make sure there isn't a subsequent accident into the first one. You don't want to be another casualty. Remember this is NOT your emergency. Protect yourself!

In the scenario try to stabilize their spine; DON'T move them unless absolutely necessary and even then stabilize their cervical spine. They may not bleed to death but won't be happy if you turn them a quadriplegic by moving them.

Ensure Airway, Breathing, Circulation, and control HEAVY bleeding. Most wounds can be treated while the person is still in the vehicle. Only worry about life threatening injuries like not breathing, cardiac arrest, or major bleeding. The others like broken bones and minor lacerations can wait for treatment.

If the person is conscious and talking find out about medical history, medications, and drug allergies. The rest isn't as important if they lose consciousness.

In the FAK I suggest a minimum of: 4 pair gloves, safety glasses, medical mask (again protect yourself!), Sharpie (write on your forearm pertinent information to pass on), a micro headlamp, shears, 6 Roller gauze, 8 4x4 & 4 5x10 gauze pads, 1-2 Ace wraps, Medical Tape, 2-3 Triangular bandages. With these supplies one can bandage just about anything on the human body. In my kit I've even got a 10x30 multi-trauma dressing and with that I CAN dress any significant wound. My kit is actually double the quantities in this kit, but these supplies are where I would start.

Have in vehicle saniwipes to clean up with afterwards.

One can add Quickclot, tourniquets, and Israeli dressings but they aren't needed in the majority of situations and are expensive comparatively. The Ace wrap, gauze pads, roller gauze, and triangular bandages can do the same thing. I've got them but already had everything else first.

The first thing one should do is get first aid training from a good source. If you've got equipment you should know how to use it!