Ok, I opened it up and did some testing but will need some help.
I'll post what I find in as much detail as I can in case that helps someone else!
The Problem:
* Tailgate Window goes up *intermittently* (but never before a rainstorm!)
TG Window goes *down* 100% of the time.
Light Blue wire sends power to Window motor to go UP.
Brown w/white stripe - power to window motor to go DOWN.
I have a manual for a 73 and 91, but could not find a wiring diagram for the Tailgate!
Assumption #1
Safety Switch is bad.
(Safety Switch prevents the window from going UP unless the tailgate is closed.) Reading on the 'net that this could be the culprit and the symptoms match.
Test 1:
Bypass the Safety Switch. Sounds easy right?
Here is the switch:
View of the latch and switch mechanism:
Conclusion:
By-passing the safety switch did not change anything.
The window only goes UP intermittently.
Other things I tried:
Plugging each Light Blue wire (that would normally go to the Safety Switch) directly into the window electric motor. No change.
The assumption here is that ONE of the lt/blue wires should have power.
Normally the Brown/white stripe wire is plugged into the upper tab on the electric motor ('upper' when tailgate is open).
So I swapped the Brown/white stripe (which has been reliable) to where the lt. blue wire plugs into the electric motor (UP).
Then I used the key switch and the window went UP.
This means the Brn/wht wire and switch (down position) is reliable AND the motor is fine.