Well, switching the two wires seems to have worked -- mostly. It sill sporadically locks the hatch (not every time!) but the doors stay unlocked.
I found the block under the dash but I haven't tried to splice into the wires yet -- as you can imagine, it's hard to reach, so I've been unmotivated to try. Maybe when the weather cools off a little. I'll post the results when I have them.
Thanks again!