Cabby Horns not working

I have checked the ring and fuses etc.  All appear to be good.
I also earthed out the main nut in the centre if the steering wheel - but no joy.
What should I try next?

Well the horns have 12V all the time, and you are switching the ground to them.  So may I ask if the horns work if you apply 12V and A ground to them?  Horns do go bad.  So if the Horns work, then the relay is the issue.  It is usually a standards 53 relay.

From a prev answer;

When looking at the horn remember VW wired them differently from the rest of the automotive world. Most are earthed to the chassis where they are fixed and pushing the horn connects +ve 12v.

With VWs of this age they are feed with +12v when ignition on and pushing the horn on the wheel connects to earth.

Sometimes there is a bad connection at the steering wheel, there is also a earth wire that connects the steering column/rack to the bulkhead. Unfortunately this located is in the engine bay quite low down and gets corroded and drops off.

If twin horns (and single?) there is a relay in the circuit too. As the wiring ages and resistance builds up not enough power gets to the horns to make them sound. A quick fix is to disconnect one horn leaving enough juice to power the single remaining horn.

Thanks All!!!!
It was a the 53 relay not fully seated.
It must have been me when I was checking the fuses for another fault