Well, I got mine sorted, but what a pig!
As the Passat alarm horn didn't work, I splashed out on 8L0 951 605A from a breakers (with a 3 month gurantee). Stripped out all the 'stuff', plugged it in and - not a peep!
Further investigation (and investigation in that space is no joke) revealed that the corrosion on the original alarm horn pin had spread to one of the contacts in the plug. The corroded contact had expanded and split the plastic surround, so when the horn pin was pushed in to the plug, the contact just moved backwards and no electrical connection was made.
Well, after a couple of proclamations such as "gosh" and "goodness me" (or words to that effect), I came upon a solution.
For some reason I had kept a few bits of wire from when I was messing around with the front fog lights, and one bit had a connector that fitted the plug contact (now removed from the back of the plug) just right. So, I stripped the wire from the other end, pushed it through the back of the plug and bent it over so that it wouldn't come out.
Then I pushed the connector on the other end onto the free contact, plugged in the horn and tested it. Bingo!
Just one issue though - I still don't know if the Passat horn would have worked - but David has that now and no doubt report when he's tried it.
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2002 Final Edition S8. Ebony black with Silver Grey leather and myrtle wood trim.
Current mods: solar sun roof, 20mm rear spacers, 15 mm on front, red brembo callipers, 6k headlights, rear view camera, engine remap, alloy dash dial rings, alloy navi rings, tt/phaeton pedal upgrade, (and custom matching foot rest) dension ipod interface & parrot hands free kit (both fully hidden), av input, tv in motion switched thro' PF switch in blanking plug right of steering column, Audi 'quattro' sill covers, repositioned centre console switches, radio clock, .
Planned mods: auto-dimming rear view mirror, dash cam (as steamship's), fit the ski hatch, refit philips drl's (or maybe not - nope, definitely not - horrible botch!).