Do you know if the wiper goes to the LED or one of the terminals goes to the LED? Essentially you want to know if the trimmer is used as a variable resistor.
The wiper goes to the LED. It's shown right there in his original post.
If the trimmer is used as a divider and the wiper is at the top, you have just shorted the power supply.
And you probably burned out the trimmer just before you got to the top. But it does indeed turn the backlight off, either way!
That requires some resistance on the led ...
Which should be present in any design including one using the unmodified shield. There may be one on the PC board that holds the LCD, but don't count on it.
If the timmer is wired in right, you can use it. Otherwise, you can cut the trace and wire in a resistor.
That is essentially what he proposed in his original post except he planned to remove the trimmer instead of cutting traces.
Don
EDIT: Wouldn't he be better off leaving the trimmer in place, cutting the trace between the top (right hand end) of the trimmer and the 3.3v pin, and moving his resistor connection over from the wiper to the top of the trimmer?