I hope you have already figured this out by now. Sorry this is so late but I think this might help out some others.
I just fixed my trucks 3rd door cable problem and it only cost me a .20 cent 1/8 ounce bullet head fishing weight and a little time to figure it out.
First, since the door is probably still shut, scoot the passenger seat forward and also tilt the seat to allow room to work.
Pull on the top half of the door panel as it is press clipped. This is a 2 piece assembly, and then remove the 2 screws from the window lock on the third door to remove this top section.
Then you will see 2 screws and one nut on the top of the next section to be removed. After that pull the bottom panel upward as it is hooked by 2 prongs and then you should be able to see into the door after this 2nd section is out of the way.
Look for the top cable which is the problem area most likely and pull on the cable while also using one of the 2 door handles to actuate the bottom latch at the same time. After a few minutes you should be able to figure out how to open the 3rd door.
Once you can work with the door open you should be able to find the problem and it most likely is an inherent problem from factory with the top cable assembly as it is bent unussually at the point where it is slotted into the cable and lever assembly. This most likely has caused the cable to break at the slotted portion of this top cable. ( The bottom cable is in a more appropriate bend which caused no stress at the same point of the slotted portion )
Once this is discovered it is essential to remove the swivel clip holding this cable to the lever assembly. Be careful to just pop open the clip and not to remove this entirely.
After the cable is free you should have enough room to maneuver this to a spot where now it becomes critical to observe the mechanism's operation and the distance required to cut and remove the threaded portion of the end of this cable. You should only need to cut about 1-1 1/2 inches to achieve this.
Now with a bullet head fishing weight which needs to be drilled out to an openning of 5/64", slide the bullet head weight to approximately 1/4" from the broken portion of the slotted casing portion of the cable taking precaution to pull the cable before cutting it to be sure it is sleeved properly and then crimp the bullet weight sufficiently with a pair of channel locks a few times to attach the bullet head to the cable. (this portion takes special attention to the distance required to accurately position the weight in just the right spot before crimping)
Now insert the cable through the swivel with the bullet head on the opposite side of the clip and snap the clip encassing the cable with the bullet head weight on the opposite side of the clip and then test it out.
If measurement was done properly you should only need about 1/8" of cable to actuate the top latch and you are back in business.
Reassemble the door panels and take pleasure knowing you just saved yourself $69.00 in parts and who knows how much in labor cost.