very few cars have keylocks on the BACK DOORS any more. your 2003 outback must be a very rare model.
all the parts, the keylocks, the inside lock toggles, and the solenoids, all move the lock mechanism - when that's frozen solid, nothing moves. so the first step is to thaw out the locks. this is significantly more difficult on the rear doors, where there's no keyhole to stick warm things in. sometimes, flipping the power doorlocks a few dozen times will work them loose, hopefully before the solenoids burn out. and applying a hair dryer, and engine heat, and some road vibration, will free up the last door or two.
i would guess that the culprit is ice, either as frost or water infiltration (we had some freezing rain), and the answer is to dry it out as much as possible, and use lubricant to displace any remaining water vapour. (as a previous poster has pointed out.)
what i'm really after is tips on how to lubricate the locks, preferably without stripping the whole door, and on which lubricants are recommended or work the best.
but this newsgroup has the most astonishing information - i'll be sure to keep my eyes out for '03 outback doors the next time i'm at the wreckers!