Hi David, Most Xantia's have alarm boxes under one of the seat's, fastened to a plate on the inner sill, usually you can just unplug them, but if there are two seperate wires in a 2 pin plug, they will need joining together to bypass the immobiliser. ( I don't think that yours will have that as it's got the other immobiliser.) there is also a relay under the column on some of them that you can unplug to stop the lights from flashing. H.T.H.
P.S you might be able to here the alarm box clicking when the lights are flashing. ??