There is no way to reprogram the BCM.
Cut the green wire of the outer chamber at the "junction"(yellow). Put a new pin on the wire. (Bosch service sells them). Place the pin in the connector of the light. (so the now 4 pins form a square in the connector)
That's it.
The outer chamber will work as a turn signal, the inner chamber will work as a stop light. There is no way to use both chambers as turn signal and stop light in the GT as far as I know.
And a little personal "hint":
Mount some red reflectors at the rear (mine are on the fascia directly arop the exgaust), no policeman will stop you for the illegal lights, but they will stop you for missing reflectors! (trust me... I know it...
)