Here is the info on the floating ground adapter which can be used to get
around the common speaker grounds used in some of our VWs.
http://www.crutchfield.com/cgi-bin/S-3QDWrdZzLfs/ProdView.asp?s=0&c=2&g=751
&I=55303FLT1&o=&a=

 It cost $60, but it might be worth it depending on how much time you have
on your hands.

                                                        Jay