I have been working on my mother's '92 740 Turbo wagon. It has a short somewhere in the #5 fuse. A BAD short. But after working on it and going through about 4 boxes of fuses, I finally was able to at least discover the source. Volvo fuse boxes are pretty easy to remove and test. With a book on one of these cars, there isn't much to tracing down the electrical system. I know Volvo's are pretty notorious for electrical glitches, but this is the first car that's really had one. I think perhaps it's the radio amp.
On the car of which you are speaking...really only things like the glow plug relays and major accessories have the power to do that. There are LOTS of relays in these cars...I would suspect one of those as the cluprit.

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R Talley
88 BMW E32 735i (make it go away)
95 Volvo 960 "Inga" (valve problems)
95 Nissan Maxima (the indestructible)
89 Ford Ranger XLT (ugliest truck in the South)