I'm in the process of doing the same install with a Panasonic HU and Sirius direct adapter. After researching this board and Crutchfield I'm doing the install the way you described. I used the VW harness for the power connections, etc. but cut and spliced in some speaker wire -> RCA line converters to connect to the headunits front and rear pre-outs. Running the VW harness alone for the speakers will over-power the factory amp and also cause you to amplify the noise from the aftermarket HU. HTH - Jay