Hmm...
According to SLP, when the GRA (presumably a system or function) is active, the 12V power from SB4 is routed to the MSTG (presumably a module or controller) and the vehicle's onboard network control unit.
The light was already on before the MSTG was coded as being on GRA.
After that, the vehicle's onboard network control unit (STG) should send the message

.
So, it's a soldering iron after all.
Best regards,
Micha