Excellent find. I looked up the WRX Service Manual. The Yellow wire is from the Ignition Coil #4. It's found on Pin 8 of the E3/F60 Brown Connector. It's under the Windshield Washer Reservoir area.
Ignition Coil #1: Green/Yellow Pin 5
Ignition Coil #2: Blue/Yellow Pin 6
Ignition Coil #3: White/Yellow Pin 7
Ignition Coil #4: Yellow Pin 8
These wires go to the ECU as well.
Ignition Coil #1: Yellow Pin 24
Ignition Coil #2: Yellow/Black Pin 23
Ignition Coil #3: Yellow/Red Pin 22
Ignition Coil #4: Yellow/Green Pin 21
Waveform 13-14 volts.
Common Wires to the 4 coils are Black/Yellow & Red/Yellow.
Originally posted by brunetmj
Ok. The wire I used was a yellow wire. It is the coil closest to the driver. However if you choose another coil as long as you dont use wires who's primary color is red or black you can use it. These red and black wires have a yellow stripe and are power and ground for the coil.