The 1989 740 fuse block is behind the ashtray and under the radio.
It comes out easily for troubleshooting. Pull out the little plastic trim
around the cigar lighter and remove two screws, one a longger one at a 45 deg
angle, then pull out the plactic tray to get room to remove the fuse block.
There are one or two locking tabs to get it loose.
You should locate the right wiring schematic for your car, either get
the Volvo wiring diagram or find it on the Russian website, that you can
search for in the Brickboard engine.
Could be some wires switches at the fuse block. The fuse block was home to
many of the electrical gremlins in my 89 740. I found loose pins at relay sockets, and miswiring.

Good luck, Bill