The relays with plastic covers (boxes, really) are actually small electronic circuits that include ICs, transistors, components -- and one or two electro-mechanical relays. Thus, a Volvo "relay" is more than a simple relay.

The solder connections on the printed circuit boards inside these plastic boxes degrade with time, and the circuit functions become intermittent or dead. The most susceptible solder connections are the large ones -- those to the actual relay, those to large components, and those to the electrical tab connectors. Very often, you can see a microscopic hairline crack in the solder around the connection (use a bright light and magnifying glass).

The purpose of all these electronic components is to give the "relay" different or specialized operating properties, such as intermittent-sweep wipers, delayed dome light, wipe-with-squirt (windshield washers), timed rear window defrost, switch-on-switch-off OD, and so on.
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Don Foster (near Cape Cod, MA)