I ran into the same thing with my 1983 245GL some years back. Called my current Volvo tech and he told me that occurrs when both bulbs are burned out.

Check the fuse again. Volvo fuses have a bad habit of getting oxidized enough so that current won't flow but they look good. Rock each one along its long axis with a fingertip. That's a first action whenever anything goes wrong.

I have looked at all my wiring diagrams and have never found the interface that causes that warning beeper to go off due to brake light failure, but it does.

Good Luck,

Bob

:>)