How about the "VW Navigation Lock" that was being advertised in Europe a few years back. I don't know if its available over here or even if its still made but its an idea to prevent those feckers.
From the Internet...Volkswagen have designed a lockable satellite navigation cover for its RNS 310 and RNS 510 models.
Volkswagen designed the cover following a spate of navigation unit thefts from VW cars in Europe, such thefts becoming pretty endemic in Holland apparently.
The Navi Lock system locks a large metal plate over the entire navigation system, theoretically obscuring the hardware and making it more difficult to steal in the process. When owners are ready to use their nav screen, they simply unlock the plate using a key and go about their business.
The system is available for the VW Golf and Passat in Europe, but it’s not cheap either., costing 219 Euros including installation, but still cheaper than having to buy a replacement navigation unit.