My mirror is on the bigger side ( >22" across) so I added an LED strip on each edge. For a smaller mirror or 1/4" etched acrylic instead of 1/8", one high-density strip along one side is sufficient.

Use the screw terminals for the LED strip (or strips) and for the power and ground wires coming from the PIR sensor. Connect the +5v wires to + and the GND wires to G. 

The data IN wire from the NeoPixel strip goes to the numbered GPIO pin (19, 21, or 22) in the screw terminal.

The data wire from the PIR sensor gets soldered to GPIO 18, as shown in the diagram above. Don't use the GPIO pins in the screw terminal for this -- they are output only and won't read input data from a sensor.