Sounds like an electrical issue, could be a variety of things... Mine has what I believe is a PNP switch issue, won't start
on a good battery, getting the transmission shifter through the gears and back sometimes gets it resolved. The issue seems
to be pretty common on 960/S90s. Just a thing to ...

Have you checked the light bulbs at the back of the instrument cluster? (They usually go out gradually, one by one.) Any other issues besides the lack of illumination? Here's a link to 740 manuals (there's a wiring diagram there too). Hope that's helpful.
http://www.volvotips.com/index.php/740-760 ...