I managed to get to the top vacuum solenoid of the two that are located under the alternator. It checked out electrically, 16 ohms across the pins and eventhough it was filthy on the outside its filter wasn't as bad as the one near the battery tray.
I can't see how it's possible to get to the bottom solenoid at the alternator from above. It's bolted to the same frame as the other, but from underneath. Nor can I see how it is possible to get to it from under the car either, as the AC sits directly underneath the solenoid.
The problem still persists, but this is the end of my troubleshooting, for now at least. The ground is covered by 8 cm of slowly melting snow, so I'm not about to lift the car onto jack stands and have a look from below before the weather improves.
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