I purchased my first Volvo in January and have been lurking here and on a few other sites since. I seem to have had the typical issues with the S70 model including the power antenna mast, thermostat, squeaking steering wheel column, and headlight washer tee (I haven't fixed this one yet). I also had the rear driver's door lock that would lock, but not unlock. I found the instructions on VolvoSpeed where someone dismantled the latch and found a repair. I really didn't want to try that nor did I want to spend $100 for the part. Here's what I was able to do instead:

1 - Purchased a "generic" 2-wire power door actuator on ebay for $8 with shipping.
2 - Purchased wire connector tap-ins from Radio Shack for $2.
3 - Removed the door cover. Great instructions on Volvospeed, by the way.
4 - Identified a connector with a purple wire, yellow wire, and yellow/brown wire and unplugged it.
5 - Using a multi-meter, determined the yellow powered the lock and the yellow/brown powered the unlock. I do not know what the purple is for!
5 - Drilled one hole and installed the generic power door lock actuator just below the door lock. I connected it to the metal rod that you use to pull up/down to manually lock and unlock the door. It came with a metal rod and connector to do this.
6 - The wires on the generic actuator were green (lock) and blue (unlock) and I connected these to the yellow and yellow/brown wires using the tap-in connectors.
7 - I hit the power door locks to test it and it works beautifully!
8 - Put the door back together. I had to enlarge the hole in the plastic cover to the two rods could come through. But everything else went back together perfectly.

I posted a picture of the installed actuator over in the technical picture gallery.

Total time was about 1 hour and by using the tapins, I can always go back it this doesn't hold up and I want to replace the door latch. I did not reconnect the connector to the latch - and I still don't know what the purple wire does, but it doesn't seem to be affecting anything I can tell.