OK, here is the diagram. Take note that there are two (2) fuses for the tail gate window, one used by the dash switch and one used by the tail gate key switch. From the diagram, the only thing that would allow the tail gate switch to fully function, and not the dash switch is a bad contact inside the dash switch. As you can see from the diagram, the tail gate switch uses some of the contacts inside the dash switch to function. A new dash switch should solve the immediate problem, but then your options are:
1. Get the tailgate key cylinder out for a locksmith to re-key it. The best thing to do is to have the tail gate key cylinder keyed to match the door locks.
2. The least expensive thing to do is replace the key cylinder on the tail gate with one that has a key for it from a junk yard.
In either case you need to get the cylinder out. I'm not sure I can explain it so you can follow my instructions, but I'll give it a try
1. Lower the tail gate
2. Remove the access panel
3. Find the motor leads along the bottom of the tail gate
4. Using a 12 or 14 volt cordless tool battery, jumper the motor to move the tail gate window all the way up with the tail gate open.
CAUTION: To be on the safe side put something under the open window to support it while you are working on the switch so you don't accidental lean on it and break the glass.
5. Once you have the window up you can remove the key cylinder, it is pretty obvious how it comes apart, just don't loose any of the clips and spring washers. A nut holds the switch onto the cylinder and a spring clip holds the cylinder in the gate.
6. After you get the cylinder out,
lower the window before you shut the gate by reversing the leads to the motor and then close the gate.
7. Now you can temporarily close the window by climbing in the back and doing the same thing you did to get it open for service.
8. take the cylinder and your door key to a locksmith and they can re-key the cylinder to the door key pattern.
9. Do what you did to get it out.
Good luck,