This is a fairly simple mechanism, the biggest problem being access. If you need to fit a new bowden cable - like a bike brake cable - you have to fold back the carpet around the operating lever by the driver's seat . The lever mechanism can then be removed - two screws - and the cable itself disconnected from it. To follow its route through to the filler cap requires removal of the rear seat - awkward! - and the trim panel on the righthand side of the boot. The cable then disconnects from the plunger by turning it clockwise.
In what way does the mechanism seem broken? Looking at your photo, it may be that the problem is really only one of gunge. That area ought too look like this
The square black plunger should pull back when the lever is pulled and then the leaf spring arrangement should push the door open. It should also depress easily with finger pressure. You could try a good clean up and some lubricant that might just help you out of a longer job!