SAM or Signal Acquisition Module is basically a controlling fused relay unit, their is one in the boot (rear SAM) and one in eng bay (front SAM).
They control things via CAN signals, like boot release, turn signals, fuel level, brake lights and so on.
When the battery is replaced or jump started without precaution, a voltage spike occurs, this high spike is the biggest killer of sensitive electronic components.
As far as the rear SAM failing I have opened several to see if there is any visible damage but found nothing, from what I can gather this voltage spike wipes the memory chips or roms within the circuitry, no matter what you try, you just can't reinstate communication and it has to be replaced.