Hi Bill:
After the 04-15-2008 post a number of new reports appeared about problems with electrical functions in the rear hatch, involving the wiper, the back-up lights, tail lights, and yes, the lock. So there was a pattern, and for the most part it was traced to broken wires in the flexible boot between the car body and the hatch. That pretty much superseded the need to get to the BIU. (Most of the wires go through the flexible boot on the right side. A few go through the boot on the left, but this does not include the wires for the lock.)
In the case of the hatch lock, as noted, two wires are involved, the WB and the LgY. Both have to be intact for the latch to lock and unlock, but the power that is used to run the actuator is on one or the other at any one time, depending on which direction it's to run.
In your case you apparently have the power on the WB, but not the LgY. That suggests that the LgY wire could very well be broken.
This thread (
http://www.subaruoutback.org/forums...6-problems-maintenance/20718-rear-wiper-does-not-return-park-home-position.html) has pictures of the broken wires, as does post #12 above and the link in post #11 above.
Here's another with the same problem as yours:
http://www.subaruoutback.org/forums/80-electrical-electronics/26165-rear-hatch-does-not-lock.html. It also has a description of how the wires could be repaired (post #14.)
Sometimes the wires aren't broken at the end of the flexible boot where they can be easily seen with the boot pulled out of the car body; instead, they're broken part way up in the wrapped bundle of wires that goes through the boot. This was discovered after one member pulled at each of the wires from one end and several came out.
So the focus here would be the LgY wire. However, I wouldn't be surprised if there's more. In most of the cases I can recall, one broken wire was enough to get the attention (due to the malfunctioning wiper, lock, or lights), but more than one wire was actually found to be broken, or on the verge of breaking.
Have you checked to see if any other electrical functions in the hatch are not working? (back-up lights, license plate lights, wiper (does it park properly), rear defogger, and high brake light.)
Incidentally, in your post a passenger door is mentioned. Is this a second problem?