the horn is probably dead
This^^^
First thing, get it scanned, will probably reveal the siren has a fault.
Grab a second hand one off ebay, about £25
Here's the part number: 1K0 951 605C
Drivers wheel off, arch lining out, siren is up in the top of the arch, it is rivoted on :( so needs to be drilled out, if you do it carefully you can reattach bolts or cable ties when fitting the replacement.
Doesn't need coding in.