Need help - 1968 Dodge Dart convertible - door won't open!

We're working on the reassembly of a 68 Dodge Dart convertible. We installed the passenger door shell on the body - no windows or anything else, just the lock mechanism. Tried to open the door today and it won't! The rods are properly installed, and appear to be properly actuating the door latch - we compared it to the driver's door. It all looks good from inside the door. but when we try to open the door it opens about an inch - to the 'partially closed' position, but it's stuck after that. The hook mechanism inside of the latch body won't open and allow the striker pin to pass.

Without taking one apart, I don't understand the mechanism enough to figure out if there is a way to unlatch it from inside the door. Since we can't get the door open, we can't remove the latch to fix it! Anyone figured this out? Anyone have a detailed drawing of the latch that shows how it works internally?

Thanks!