What seems to be some defect in the connector terminal tension or the "circuit board" most often stems from moisture. The moisture invades that area through cracks in the lens-to-housing welds or cracks in the lenses themselves, causing invisible oxidation in the electrical contact between the copper of the circuit foil and the silver inlay of the terminal contacts. This causes the contact resistance to rise, creating local heating, and in turn, further increase in resistance melting the housing's black plastic tongue around which the polyester circuit board is wrapped. The contact tension squeezes it thinner, further reducing contact and melting from resistance. Sounds like it cooked your harness plug body, too, the 949542 contact housing.

The aftermarket lamps have a disadvantage that their lamp socket spring contacts have an inferior plating compared to the Cibie/Valeo OE lamps, so the black plastic lamp housing behind them distorts even more quickly, but even the $300 lamps you get from Volvo now are poorly made* (in my sad experience) so there's no piece of advice that will provide lasting tail light satisfaction except, perhaps, to wire them directly to the lamp sockets and drill drain holes in the lenses, if you can bear the insult to the original design. The lamps you buy now are not made in production-line quantities with factory quality control.



Above is an unclear close shot of the tongue of plastic melted and distorted by the heat of the harness connector oxidation problem.

Below is another of the same process occurring at the lamp socket. The circuit board gets incorrectly blamed for this.



And this is the only way I ever got years of reliable tail lights in a 240:



*I've returned broken lamp shipments to Volvo until getting a set without loose lenses or cracks, but even those leaked within a few short years. I guess they just don't have the same plastic welders on the short-run lines building these 30-year-old designs for replacement parts. And why would they?

--
Art Benstein near Baltimore

Watching the sunrise outdoors statistically increases your odds of having a good day. And needing a nap after lunch.