The classic cause of the problem is the cable sheath "stops" on either (or both) ends of the cable sheath crumble away and when you try and open the door, the cable sheath simply sides in the latch bracket and the cable itself does not pull on the latch release.
Probably the easiest thing is to remove the pull handle, it's simply a type of rivet you can drill out (3/16" I think). Center pin can be pushed out with a punch and the aluminum rivet is very easy to drill.
With the handle free, you can pull on the upper cable end and encourage the latch free. The upper cable routes to the lower latch, the lower cable to the upper latch.
Best of my knowledge, the cables are not available, but you can use bicycle cable barrel adjusters as substitutes for the original sheath stops.
The latch ends are fixed in place, the handle ends have small plastic clips that allow assembly. You'll see those when you get the handle off.
Be warned, once the first cable sheath stop crumbles away, the other 7 are likely not far behind.
