I've had a Cobra on my mk3 for 8 years with absolutely no problems or maintenance whatsoever, it really is "fit and forget".
The mk3's didn't come with factory fitted alarms unless specified, however the post 1996 models did have factory fitted immobilisers.
Who would steal a mk3? I've had many attempts (all unsuccessful), but I guess that depends on what your Golf is like.
Mine is up for sale now, with much regret, due to a growing family.
A Cobra 7928 was the best alarm/immobiliser I've ever had fitted to any car.
