The green tab is already on the back of the HU, just needs unplugging and plugging onto the techincs harness - iirc you are using a techinics harness?
no need to buy anything
It is a bit of pain in temrs of having to remove the HU out and getting to the back but actuall swapping over is not bad 2 min job
The SOS the footwell trim has to come out - its only held on by 3 or so screws and just slides down and have a look if the sos connector is connected - if it is probalbly just needs error code resetting
see my link for green tab and sos connector further down
http://www.e90post.com/forums/showthread.php?t=769602