Locate the alarm module or horn unit under the driver seat by removing the seat bottom cushion with an "E" Torx driver (VW calls for a T-55 screwdriver, which should also work with a T-50). If that does not help you find it, locate a black wire with white (WHT) and two brown (BRN) wires, which lead away from the sensor to the alarm horn behind the left/driver front inner fender liner by the steering rack in that general area.
With one end unplugged, the alarm feature will continue to function as soon as you lock the vehicle as per normal use with doors completely closed with a working key; that particular motion sensor's part of the alarm system's function becomes disabled or inactive until the black WHT/BRN plug end gets secured and snapped back together.
