Depends what kind of immobiliser it is. Most aftermarket ones operate by cutting two circuits, usually the starter crank and the fuel pump. Whip the bottom of the dash off, find the immobiliser box (trace the wiring back from the immobiliser key socket or antenna) then check out the wiring. You should be able to see where they've cut the wires and interrupted them with the immobilser wiring.
Without knowing the type of immobiliser it's hard to be more specific, but most operate this way. I used to cut the starter feed at the ignition switch loom and the fuel pump at the back of the fusebox.
Graham., May 2007
