2015 WRX:
Tach Signal (green wire of shift light): to ignition coil (coil #1 blue wire with yellow stripe).
Ground (black wire of shift light): to white wire of coil #1
Switched 12 Volts (red wire of shift light): to brown wire with white stripe on coil #1
Set the shift light to 0.5 PPR (1 cylinder mode)