I think you are right that a ground problem is the most likely cause for the behavior you are seeing. Maybe got a corroded connection from the moisture in the interior, or it could be unrelated and just old or loosened up. Either way, same process.
Not having much familiarity with the 200 series electrical system, unfortunately can't say where you would want to look first. Perhaps someone with more 240 knowledge will be able to chime in. But the Volvo greenbooks I have for the 7 series do have a section about grounds that shows all the ground point locations around the entire car. Maybe would be good to find a 240 greenbook for electrical system and look for the same thing?
Other than that, if I had to guess where the ground locations would be, I would start by looking under the dash around the firewall and the kick panels on both sides. Seems like that is a common area for harness grounds either way. Maybe the transmission tunnel console/handbrake area also.