Ok, my thoughts. Please feel free to fill in with info or vids.
There are 2 scenarios that come to mind. Shooting while driving or getting run off the road and the shooter getting out and approaching the vehicle while shooting, or approaching then shooting.
scenario 1, while driving. I would expect that the shooter would try to pull parallel with the driver as they would most likely be trying to shoot through through their passenger window with the window rolled down so as not to shoot their window. Most people, even when every agitated, default to basic actions and will not shoot through their own windows or windshield, and will roll a window down and and shoot through the opening. Also most people being right handed, will try to shoot across the passenger seat though an open passenger window meaning they would need to be parallel with the drivers vehicle.
Therefore armor behind the seat would most likely be ineffective, where armor in the door panel might catch some poorly aimed shots that when low into the door panel, in a, while driving, type scenario.
Scenario 2, run off the road, or in heavy traffic, is harder. With the shooter getting out of their vehicle it gives a LOT more shooting angles and can really vary, but I would anticipate a quartering angle, a 45 degree with the shooter approach on the drivers side. Most likely, based on the road rage vids I have seen the shooter would approach the drivers window and start banging while yelling before shooting. In this instance armor on the back of the seat, or door panels would be useless as the shooter would have a clear shot through the window avoiding all armor. If the shooter got out and started shooting right away while approaching the vehicle, is what I would consider least likely, while possible, and the only one in which the armor might help.
If it were me. I would want armor in my door panel, and a quick deploy soft shield I could carry. The door would cover the side, and if opened the front quarter of the vehicle. The shield when deployed would cover the rear quarter of the vehicle with the door open, or would offer double protection if it were coming from the front quarter. Or could be pulled up to to cover the window.
As Law enforcement I was always trained that, in a gunfight, a vehicle is a death trap. That in a fight you need to get out of the vehicle ASAP and get a gun into the fight ASAP as the vehicle itself offers little to no protection, and only concealment, not cover. This is also wearing body armor, but there is still a LOT that can go wrong.
As I said, just my thoughts on what I would anticipate in a road rage incident and why. Feel free to critique, refine, or counter them, it is always good to get fresh ideas and perspectives. There are also lots of road rage and road rage shooting vids to watch and learn from. I was watched a few but no where near all of them.
Hope this helps.
P.S. In relation to armor types. Kevlar or fiberglass would be my recommendation as they catch the bullet. Steel armor is great, however if it were on the back of the driver seat or in the trunk, you have to deal with the ricochet issue. If there is anyone in the back seat, if they were not already hit, they would most likely get hit with lead fragments. If in the trunk there is the possibility of it getting redirected into the gas tank. While this would most likely NOT ignite the gas or cause an explosion, it would drain your fuel and leave you stranded.... so I would say NO to steel armor, unless it was on the outside of the vehicle
P.P.S heres a "how its done" vid for how vehicle are professionally armor plated. There are others that I have seen. While not exactly what you need, it might provide some insight, or give some ideas on how to accomplish your goal or protecting the driver.
https://www.youtube.com/watch?v=ezjCixEeb3s