Here's what happened: the spade contacts on one of the license plate lights were touching the sheet metal because the tabs on the back of the black plastic housing weren't inserted properly. Why this short did not occur sooner I don't know, just luck I guess. The angle of the contact lugs and the tight clearance of the tabs was a perfect setup for this exact thing to happen. I had a really difficult time pushing in the back tabs to engage properly even after I discovered the problem by removing the plastic cover on the trunk lip. (Car has only been driven in daylight since the problem occurred.)

So now the left front and left rear parking lights are working. I bent back the contact lugs on the current Scan Tech units and removed the bulbs so I can be sure the taillights continue to work. Now I will buy real Volvo license plate light housings so at least this isn't a future problem-and maybe wrap the contact lugs in tape as well.

Now for part II: I have to get my hands on Volvo E-code license bulb housings. Those have a plastic bayonet base into which the bulb fits. The contact lugs on the bulb holder extend up through the center of the opening in the sheet metal. Well out of the way of any other electrical path.