Hi,
Did you get anywhere with this?
I just replaced the disc horn on my '89 vintage 90. On the one I took off, there was a single spade terminal for the single positive feed.
Both the replacement units I bought (disc and "snail" type - having bought the snail first based on a part number look-up rather than actually looking at the car) had 2 x spade terminals.
Ended up fitting the snail behind the grille no problem. Made an earthing strap to the single nut/bolt mounting point. All good.
Appreciate none of that helps you move through the diagnosis list above but you may find a similar wiring arrangement.
For me, all is now well - and I've got a spare disc-horn unit still in its blister packaging.
If you (or anyone else) wants that, email me. A stamped/addressed A5 jiffy and a pound coin will do!