It's a royal pain in the a. You have to roll the back window down almost all the way so you can reach in there. Then you pry at the edges of the INSIDE rubber grommet/strip that is up against the back window. This is a sort of rubber strip that is fastened in the frame by little metal slits, but once you get the edge up, you can pretty much pull the rest of it out carefully. This lets the back window swing and rest against the inner plastic moulding. Now you are able to see down into all the crap in the door frame, and you can see the door latch and components. You also may need a flashlight and may have to turn your head sideways and close one eye to see down in there.
The problem I had was that somehow the handle latch was missing the actual unlatching mechanism inside the frame. So i used a wooden dowel to just get the door open.(You can slide it down there and manually press the door latch without the actual handle)-- It is hard to fix the problem of the missing latch this way....this is simply getting the damn door open so we can get the inside shell off the door.
Once you finally open the hatch, it is possible to take the inner plastic shell off of the door. Just stick a screwdriver under the plastic edge and pry up. It is attached with little clips and comes out when pulled. At the the top, up by the back window, there is a lip on the plastic over the metal door casing.
BEFORE YOU MOVE ON, ROLL UP THE BACK WINDOW AND GUIDE IT WHILE DOING SO... BECAUSE REMEMBER IT IS NOT ON MUCH OF A TRACK!
Once you get the plastic off, you can see two different metal panels that can be removed with a wrench. The BIGGER compartment will let you get to the door lock and latch.
Now once you get the big panel off, all the guts are visible, and you can do what you need to fix the problem. Mine got fixed when i simply put the latch back onto its proper track.
Hope this helps...I actually damaged the outside moulding on mine because I had no idea how to get into the door to fix the problem. Once I realized that I could do this, it became much easier.
But it cant be that hard, considering I dont know that much about 4runners yet