I do have some sympathy if the car is a genuine real Land Rover because these things get nicked regularly, unfortunately because the alarm is going off regularly, Boy-Cried-Wolf will apply and no one takes a blind bit of notice of the alarm other than try to ignore it.

He would be better having a professional standard immobiliser/tracker or one of those clever ones which allow the engine to start enough to get the vehicle moving only for it to cut out seconds later with the horns from hell and lights going like billio drawing lots of attention...whether you could persuade the owner of the value of such a fitment is the question, i certainly would if i owned a vehicle so easily stolen and desirable to thieves.

If it's a Defender, they have considerable air gaps to the outside when the vents are closed!