Got a solution to share. The V70's inside driver's door broke a while back. I scoured the web looking for ideas, found a few, and got started trying to make something work. Apparently mine had been "fixed" once before, as a piece of wood and a large binder clip fell out when we got the door panel off. Not sure what it was doing....
As I got looking at it, it was clear that the receptacle in the door panel was broken in the front and on one corner, letting the cassette with the cable pop out and causing the door handle to stick out. It worked, but it was clear it wouldn't last. I guess the binder clip prevented it from popping out entirely until we got the door panel off.
My solution was this: I had some leftover 1" square steel tube, and I cut a piece about the length of the cassette, and then cut one of the sides off the tube, leaving a 3-sided square. I carefully pulled the sides open further with vice grips so that it would fit snugly over the receptacle. The steel held the cassette in place, and magically it was fixed! I also used some "steel stik" epoxy putty to make for darn sure it wasn't going to pop off due to door slamming or other reason. See below for photos!
The above shows how it it fits. I was test fitting it. That took a little bit as I didn't want to bend the tube too much apart.
Above, the fix is in place, with cassette reinstalled.
A photo showing the epoxy putty I applied. I also put a little scrap of sheet metal in place to help shore up where the plastic of the receptacle was broken. Not sure if it makes any difference, but it made me feel better.

Hope this helps others with this common and expensive problem. It still blows my mind the Volvo fix is to replace the door panel...
Best,
Bert