I used to "carry" in my passenger seatback pocket for years when I owned a Firebird that had one. It was a very cool place to stash the gun as long as there were no passengers in the back seat. When there were, I would just wear the gun strong side like always.
At the risk of getting the tactically oriented guys irritated with me again, if you should have passengers in the back seat area of the truck, just carry strong side and "risk it." In my view, when you're in your vehicle, your first line of defense is, as always, your brain - i.e. situational awareness. Your second line of defense is the combination of the gas pedal and the brake pedal. In almost any situation, these two aspects of your layered defense should give you time to access your third line of defense - the gun - no matter how you are carrying it.
You notice a threat developing, get the heck out of there. This includes thinking ahead far enough to avoid boxing yourself in at stoplights. It includes watching your mirrors and your overall surroundings at drive up ATMs and such other similar places.
About a year and a half ago in Austin, I had a car full of what looked like banger types cut me off badly (twice in quick succession, so I knew it was not accidental) on a frontage road. As they went by I could see their angry expressions and gestures directed at me. I have no idea why they did this, as I had not impeded their progress in any way. It might be that they didn't like the "W" decals and the NRA decals on my back window. I don't know.
Looking ahead, I saw that we were both going to be sitting at a redlight in a moment, with me about two cars ahead on their right. There were 4 or 5 guys in their car, and just me and my wife in my compact pickup. I was carrying my Commander strong side at the time. I slowed to give myself some extra time before passing them, drew the gun, and held it in my right hand on my lap. As I cruised past them slowly (more slowly than I needed to) I stared at them and saw more angry expressions and gestures. As I pulled up to my spot in the traffic line, I kept an eye on them in my side mirror. I can remember clearly thinking to myself, "Don't get out of that car."
They didn't, and we all went our seperate ways.
I know it's just an anecdote, and anecdotes don't prove anything. I just recount it here as an example of situational awareness, and the huge tactical advantages it can convey. The quickest draw, after all, is when you alread have the gun in your hand, right?
If those guys had gotten out of their car and approached me in a threatening manner, maybe carrying clubs, edged weapons, or even guns, however I was carrying my Commander would not have mattered in the slightest, because my awareness had already worked to place the gun in my hand.
Anyway, passenger seatback pockets work great. I found myself that the pocket tends to sag after a while, but any good auto unholstery shop can take care of that when needed.