Well, I'm not only an electronic type guy who's always played with electical devices for fun since being a kid but I'm also a Volvo dealer master tech and no dealer tech would do this attempt at work for various reasons other than what I've previously mentioned.
Volvo now wants the old units back to apparently be rebuilt(?) when they didn't use to before and that's why I've dissassembled a few of them to check this idea. I'm sure that IF the problem really is bad solder joints (which to me is purely a guess here), someone could fix it if they're experienced enough and had the proper equipment BUT my point here is that this idea is definately not one that I'd say just anyone could do with basic store bought soldering equipment at home.

If I owned a '98 X70 that needed one of these, I'd just prefer to buy a new unit myself after seeing what's inside.

Another idea about this is if one trys and fails and ends up buying a new unit from a dealer, they will probably not get the core charge returned when the parts guy sees that the originals been apart. Just food for thought.