[don1952]

AFAIK it will be for coding keys to the OEM Subaru Keyless Entry system and should be left disconnected.

Wire colours B - Black; BL - Black with BLue stripe

 

[don1952]

Cheers Don!

Is there a source for that image you posted? I don't think i've seen anything approaching a wiring diagram for the JDM STi forester before.

Those are from the MY04 European Manual.

I doubt if you'll see anything for the FSTi specifically.