I installed an inexpensive aftermarket alarm a week ago...by AudioVox purchased at my local Auto Zone, only $35, model Rampage AA-929. This works on voltage drop with a 5 minute arming delay to allow for the electronics of the car to stabilize and an adjustable shock sensor with warning chirps if someone touches the car. So far, no false alarms.

I know it isn't as fancy as the official MINI alarm but it is a good alternative...drawback: has a separate FOB on your keychain. I installed it at the back of the engine compartment and have the bracket drilled into the black plastic housing just to the left of the brake fluid reservoir, and half way down giving about 1 1/2 inches between the alarm and the engine.. very easy to wire and install, tho I had to buy a flexible drill bit chuck to get my drill in between the engine and that housing