Chris STo try out the syncro awd system you need a patch of grass and a friend, floor the throttle from take off so the front wheels spin, the rear wheels should also spin after a short 1/10th second timelag, it will either work or it wont,
However here is the official method
Checking your rear diff locking mechanism.
Its for a Passat G60 Syncro, Which I am sure is the same as the Golf Rallye/Syncro.
Differential lock mechanism, checking:
(rear final drive installed, and parking brake released)
– raise vehicle on hoist
– spin rear wheels by hand, in forward direction
Note
The rear wheels must spin easily, without resistance, and the driveshaft must not rotate.
– spin rear wheels by hand, in rearward direction
Note
As the rear wheels are spun, the front wheels must be driven in a rearward direction, by the driveshaft.
Differential lock controls, checking:
– let engine run for at least 30 seconds to allow vacuum to build up
– raise vehicle on hoist
– switch ignition ON
– engage reverse gear
– depress clutch
– turn rear wheels by hand, both forward (see Note), and backward
Note
When the rear wheels are first turned forward, the differential lock controls should engage, with an audible “click.”
The front wheels must be driven by the driveshaft, spinning in the same respective direction in which the rear wheels turn. If the rear wheels can be spun in a forward direction without resistance, and the checking procedure indicates no other problems, inspect the following:
Pneumatic system
– lines, hoses, and connections
– check-valve and vacuum reservoir, near left front wheel housing
– diaphragm in vacuum element
Electrical system
– Wiring and connections, especially at transmission switch and vacuum element
– magnetic switch in vacuum element. (It must “click” on, audibly.)
Note
If both the pneumatic and electrical systems are operating correctly, either the differential lock controls or the mechanism itself, may be defective