My replacement switches (one new Meyle, one used Audi) have arrived and I've been testing them electrically and working out the logic with the help of Mark's pictures.
There are two pairs of contacts, connected to the larger and smaller pairs if pins in the plug. As the plunger is pressed in (by releasing the brake pedal) the shorting plate moves from the larger to the smaller pair, with a small gap in between when neither are connected. This all makes sense as the larger pins are the brake light circuit.
I imagine it's not that difficult to pop a suspect switch open (maybe warn it up a bit first to soften the plastic case?) and clean up the contacts.
Mark's photos show the corroded contacts are those away from the internal spring which would indeed be the brake light circuit presumably carrying a significant current for the brake lights.
Incidentally I've tested the internal resistance of the brake light circuit through the switches I have and both (old and new) are around 2 ohms. It will be interesting to see what the one in my car is, but that measurement will have to wait until I replace it.
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
Last edited by HPsauce; 31st January 2020 at 11:53 AM.
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