Al:
As I mentioned in my first response to your thread, I'm working with information pertaining to the 1988 760 series, nothing more recent. Having said that, I'm cetain that there's no (normal) electrical function associated with the Shifter Lock button. There's a two-wire run from the Overdrive Lockout switch in there; the lighting feed to the illuminated shift selector indicator and backup lights/ start inhibit switching are to the outside of the shift lever column proper, right below the lighted indicator panel. This stuff all comes out of the Haynes manuals - 240 and 760 series issues; unfortunately I never got around to acquiring the green book for the 760 wagon. The diagrams and descriptions for the specific models, as you probably know, leave something to be desired, but there's some information to be had. I would suggest as well that you do a search, both in the search box at the top of this page, AND on Google ( because I've found leads to many other sources of information which unfortunately don't find their way to this forum): try "Volvo shifter lock" or the like, and I expect you'll see several hits to peruse. You might even find a source of cutaways as well, but if not, let me know if you'd like a scan of the "typical shift lever assembly" cutaway from one of the manuals here. it'll give you an idea of what's going on under the transmission hump in terms of electricals, and what you might be up against if you go after this snag seriously.
I was exploring a problem with the shift lockout knob in my '98 wagon recently, and found a fair bit of info out there at that time; that's why I'm suggesting a search: it's just that the stuff can be hard to find. As well, don't forget the FAQ - there's a short article about re and re of the shift lockout button itself which might interest you.
Garth