Well, I got me tunes in the back now.  It was a mightly struggle but clean
(?) living prevailed.  My advice is do NOT try to run an
electrician's "fish tape" from the underdash area up through the pillar.
The tape's hook/loop got stuck (I didn't realize the tunnel wasn't
completely sealed) and I had to remove one side of the headliner paneling
to unhook the fish tape.  Some thinner wire (which was more flexible than a
fish tape) sufficed, and I was able to run a new right rear speaker ground,
re-mantle the interior, and all is well.

Now onto my first CV joint repack job in many, many years (didn't have a VW
for a while).

Why are they called "constant" velocity joints, anyway?  What is constant
about the velocity, as they spin at the same varying speeds of the
transaxle?