The principal behind "sacrificial" anodes is metals will tend to give up
electrons to other metals whose affinity for electrons is greater.  The 
electromotive series (a list of various metals arranged in decreasing order 
of their tendency to pass into ionic (wandering atom) form by losing electrons)
for major metals is: silver, copper, tin, lead, iron, aluminum, zinc, magnesium,
and potassium.  Galvanized metal uses this principal as zinc is more readily
oxidized than iron.  Whether using zinc as a sacrificial anode will aid in 
slowing the lose of iron ions from leaving your beloved VW, I'm not sure.  I 
use zinc washers and bolts when ever I can.  WIth a boat, the "communication"
for valance electrons among metal components is much greater (it's partly under
water) than for your VW.  It couldn't hurt.  I know jc whitless sells powered
sacrificial anode units. Sorry about the run on ... probably more than u wanted
to know.
christian