There is a small spring in the latch mechanism that is prone to breaking. When it does, there is a little bar in the mechanism that does not allow the latch to unlock even when pulling from the inside handle or directly on the lock rod. Short of driving down a bumpy road with the vehicle leaned to one side or the other, I don't know of any way to get it to release. (I was fortunate that mine eventually popped open while trying to get it to unlock). Rocking the vehicle from side to side might even help.
If you knew where to look and had a good set of dental picks, very small hands, and some sort of bore scope, it *might* be possible to stick a pick up in there and pop the lock detent out of the way long enough to get the door open, but I wouldn't hold my breath on that method.
Ultimately, you'll likely need to replace the latch mechanism in that door. It's a common failure item.