newshound has brought this to us :
I suppose the immobiliser could disable the starter solenoid, or the electric
fuel pump, or the glow plugs, or enable the engine cut-off. The Sterling
Touch web site suggests they currently provide two lines of protection,
perhaps the starter solenoid and something else.

Usually, add on immobilisers are wired in parallel (shorting out) and
series (isolating) with the various circuits, to prevent a vehicle
being started. That being the case, it should simply be a case of
removing the isolating series connections and rejointing the parallel
ones - checking the colour codes and joining identical coloured wires.