The knocker didn't 'stick' though, right? It was firing repeatedly. When heat is involved anything could happen (even if it is unlikely), so I'd double check the relay anyway just to make sure the connectivity is correct. Even if it can only do one or the other, it's possible the 'station flasher' wasn't going off during the light show (why the relay would fail to engage for that one specific flasher I don't know, but..). Or the relay could have gotten sticky (physically or a problem with the circuit that drives it), and not been opening and closing fast enough, result in half solenoid firings and half flasher (you could have not heard other solenoids firing over the knocker racket) Do the flashers still work, or are they on one of the blown fuses? It's also possible that there's a 'short' that gave power to the knocker, bypassing the A/C relay, so even if the relay was opening and closing fine, the knocker would fire any time the flasher went off.
By default the A side gets power, so if the transistor locked on then so would the knocker. D42 failing would probably cause both the knocker and flasher to lock on, regardless of the transistor.
A problem at Q30 alone wouldn't cause the knocker to knock if everything else in the circuit was good.
Luckily these components are all testable without powering the system up, so unless they all test out fine it'll be easy to at least track down part of the culprit
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