What I did on my previous car which wouldn’t unlock on the rear passenger side was to simply leave it that way and not use it.
I don’t know if this is on all cars by my Pontiac has an access hole next to the child safety switch that can be accessed from the inside on the car.
Check your other door to see if you have one. It will have a rubber plug in it. If you do you can slide your hand along the side of the seat until you can feel it. Push it out and you have access to the child safety switch. If that is somehow stuck between lock/unlock that may be your problem.
I missed the Family Guy part and figured this was going to be a link to that old BlondeStar clip, more or less the same idea.
That’s how they always work, so far as I know. They disengage the outside handle. However, once the lingage is screwed up, all bets are off (but I’m guessing, at this point, it doesn’t play into it).
While I still think there’s hope, I still think the best bet is to try to wedge the panel open at the top and see what’s going on. As I said earlier, it may be just some linkage that came off or broke and can be easily fixed, and the door may even be easily opened with the small amount of access, this is also a viable option. If the OP can live without one back door opening, just move on. He’ll get used to it and when people do have to get back there, it’s easy enough to say ‘that door’s broken, use the other side’.
Take off the inside panel of the other back door.
Lock an unlock the door while you look at the components - see what moves which way for unlock
lock the door, but don’t close it.
Use the sim jim to unlock the door - it should be easy because you can see where it needs to go and how it needs to move to unlock the door. It might help to put tape on the slim jim to show how far down it needs to go.
Duplicate in mirror form the position, depth, angle etc. & movement of the slim jim in the locked door.
My 4Runner failed like that.
I had someone operate the electric door lock switch as I operated the door handle and lock knob. It finally unlocked. Now that I could get the door panel off I found a plastic guide piece had worn out. It was just luck that we managed to wiggle things around the right way to unlock it.
Looking over some web videos I found thisalong with suggestions to flip the child lock with a flat piece of plastic slipped between the door if it’s an up and down button.
I like HipGnosis’ idea. Once you get a good look at the components and can directly see what moves when you lock and unlock the door, you’ll have a much better chance to get the other door open.
So with some time over Thanksgiving and the problem not having resolved itself otherwise, I decided some more serious jiggling might be a last resort. I grabbed the door and pulled it and pushed it, and yanked it up and yanked it down, and to the right and the left, and I banged it this way and that way from the inside and outside. Every so often I gave the power locks another try. And after a while - maybe 5-10 minutes of this - miracle of miracles, it suddenly worked. 
And what are you doing to ensure the problem doesn’t recur?
Did you check to see if the child lock was partially engaged after you got it open?
I didn’t mention this until now because it wasn’t relevant, but at an early stage when the problem came up I gave the exterior door handle an extra hard yank and part of it broke off. Replacement handles go for under $6 on eBay, but you need to remove all the interior trim to replace it (it’s bolted from the inside) and I couldn’t access that. This was of secondary concern for me as compared to the door not opening altogether, but once I got the door unlocked and opened I did remove the interior and replace the exterior handle. This involved unlinking the handle from the mechanism, and also - unfortunately - detaching the lock mechanism itself and the latch too in order to allow access to the door handle bolts which were behind them.
So while my inclination would have been to just assume everything was fixed and wait to see if the problem recurred, as a practical matter I ended up detaching and reattaching pretty much the entire lock and handle mechanism. So I assume things will be fine going forward. You never know, but that’s as much effort as I’m willing to put into this.
The child safety lock was not the issue. (I didn’t think it would be, since the child safety lock is not related to the door lock or the exterior handle, but in any event it was in the “open” position.)