There are many "universal" harness bars listed on ebay to fit VW's. Like the post above of the Subi, they simply bolt to your existing seat belt attachment points and are easy to install and remove. Seat travel is relatively unrestricted but access to the back seat on a two door car would be a royal PITA.
The real question would be if these harness bars are safe and would they provide adequate protection in the even of an accident. If you are just doing auto-x and want some help staying stationary in the seat, maybe just try a cam-lock for your seat belt and if your seats are leather, some rubber rug backing like you put between your area rugs and hardwood floors will provide a lot more grip to your seat than you'd think. If you are doing track days, I wouldn't thrust one of these universal fit things for even a minor incident.
Guess my feeling is either do a proper roll bar or stick to stock. Safety is a pretty big deal to me when it comes to doing 130+ mph down the front stretch on a track day. And for daily driving, I wouldn't run a harness anyway. You are much safer with the stock 3 point.
