Grumpy_Mike:
ReducingIs this troublesome LED different from the others? That is could it be that it is a common anode rather than a common cathode.
Otherwise you need to look at the signal on the LED with an oscilloscope to see what is happening to it when you connect the Arduino.
Which LED do you refer to? The ones in the case are common anode 12V RGB, and the strip I am trying to install is 5V 3-pin RGB.
I will try to check with my DSO-138 oscilloscope but I am still learning how to use it 