Might also be worth digging around for signs of an aftermarket alarm.
Chasing shorts is always a treat... Maybe this will help!

Number 13 fuse: Hazard Flashers, Headlight Flashers, Alarm. Brake lights, headlights, highbeams, turn signals are all on different fuses. The alarm is spread across 3 fuses and a relay. Does the fuse only blow when you use the flashers? Does the fuse blow without touching the flashers? Have you trie...

The obvious thing I missed here is to completely disassemble the area were repairs were evident. From experience, about 90% of electrical repairs on cars is garbage

The #12 fuse on my 93 850 was blowing, taking out my brake lights. I went through a lot of tests like you've done. Finally, a Volvo indy shop tech clued me that the bulbs need to be matched. I checked, the tail light bulbs were not matched for wattage. Installed a new matched set and that solved it....

There is a write up on the brickboard by somebody that fixed his 940 sending unit. https://www.brickboard.com/RWD/volvo/556038/940/960/980/V90/S90/sticky_gas_gauge_heres_fix_wacky.html Next summer I need to address the sending unit on my S90. When I get to it I'll do a better job of documenting it. ...

Undoing someone else’s hacked wiring is really advanced stuff.

The S60 I referenced had no clack while turning.