If it's the relay, it is most likely cheap. Question is, where is it? Most of the relays in my car are right next to the brake pedal - a nasty spot. But I'd imagine some are elsewhere, too.

The symptoms you describe COULD be attributed to a faulty relay, but a faulty relay is pretty rare. A relay is a powered switch. You press a button on your console that is passing just enough current to operate a small solenoid in the relay that closes the higher-voltage circuit. (Keeps high current away from your switches.) So you can see that a relay could (maybe) be frozen in the "on' position for open vent. A 'standard' automotive relay is usually $5-7. There are specialty relays that can cost more.

But the motor could be the culprit, too. Since you have an 850, it might be worth the risk to get a used one from a junkyard for $10, swap it in, and see if the problem goes away. This is the avenue I'd pursue first.

Alas, I don't possess a wiring diagram (except for lighting circuits).

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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)