I need to remove the passenger door panel to check operation of locking device. It will lock, but will not unlock.

Go to Volvospeed.com. There are step by step instuctions on door panel removal and MANY other things in the maintenance section. Headed by a volvo tech under the heading "Bay 13."
Very good site for all volvo owners.
I used this info to successfully remove my S70 panels and fix a window regulator.
You must be careful removing the trim around the door open handle - see instructions on Bay 13.

Good luck.
--
Aaron

TAKE ALL THE BOLTS AND SCREWS OUT FIRST THAT YOU CAN SEE THEN START POPPING OFF THE PANEL FROM THE BOTTOM FIRST.IT SETS ON A LIP ON THE TOP EDGE ADJACENT TO THE WINDOW.ONCE YOU HAVE IT LOOSE THEN YOU HAVE TO START DISCONNECTING ELECTRICAL CONNECTIONS ON ALL THE SWITCHES.THEY ALL HAVE SNAP LOCKS SO DONT JUST PULL OFF, UNLOCK WIRE LEADS FIRST THEN PULL.GOOD LUCK.

Bugs--welcome to the "failed lock" club. If I had a dollar for all the failed 70 series lock mechanisms I would be a friggin millionare!!! Do as RLK says above. Then To "pop" off the bottom part of the panel, slide a nice stiff putty knife between the panel and the door and it should just "pop" out the little snap from the recepticle on the door itself. Once done, "lift" the panel up as the panel edge with the nice rubber window gasket has a lip behind it that goes behind the metal edge of the door. Plan on $90-$120 for window mechanism depending where you buy your valvo parts. Good luck!