I found out what the plug does...it disables the intelligent key and makes it stupid
I unplugged it today while at the lights just to see if anything stopped working...after getting to my destination, turned car off, got out and tried locking the car with the button on the door handle..didn't work..tried locking with intelligent key, didn't work..so I used my backup autowatch remote to lock.
Came back, same thing, key fob didn't work, unlocked with autowatch and couldn't start the damn car!..was having nightmares of being stranded down the coast and remembering the thread on faulty key fobs...then it dawned on me to connect the plug back to my rear view mirror and hey presto everything works again! tried unplugging it again and testing my theory and yes it made the key stupid again.
Anyone here have a S2 sedan with the intelligent key, try this out and see if does the same