Listers, just for posterity's sake--aka the archives--I finally figured out
the wire routing.  I used deductive reasoning and a listmember's
confirmative support to determine that the rear hatch actuator wiring
originates on the passenger side, behind the glove box.  It's actually
attached to the wiring for the sliding door, and then splits at that point
and runs up over the headliner to the drivers side, where it runs along the
roofline and back to the hatch.

So, all I need to do is attach my aftermarket wiring at one access point for
both rear doors--easy!  I'll place both the lock relay and wireless brainbox
in the space behind the glove box.  I haven't had any time to follow up on
this, and probably won't complete the install until later this coming
weekend.  I'll report back with some results once it's done.

Again, for those who are tired of struggling with aging OEM actuators ($$$),
this option provides a solution for not much more than the cost of just one
new actuator.  Oh, and this can also bypass the original wiring in the front
doors, which is prone to flakiness with age.

Casey
'89 Wolfsburg Edition