You can occasionally get away without the relay but not often. Anything beyond the stock Beetle horn and a relay is a must. It'll simply draw too much for a basic push button or switch and something will get hot or burn out, or it just won't work at all. I've got two horns in mine - one for me that sounds slightly annoyed, and a happy klaxon for the passenger when they see another camper.
At one time I pondered the possibility of a row of buttons ranging from 'happy horn' to 'I'm REALLY cheesed off now' but two seemed to cover the lot really.
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Thanks for advice.
The previous owner fitted an aftermarket horn that was working until last year. I've tested it and the actual horn seems to be OK. It's the switch/button on the steering wheel (or perhaps the wire running down) that seems to be causing the problem. I shall have a closer look at it over the weekend as I don't really want to add an extra switch to the dash if I can avoid it.
Check the circuit carefully. If it wasn't powered by a relay, the wiring through the steering wheel almost certainly wasn't up to the job and possibly the reason it doesn't work now. You may well find that if you give the aftermarket horn its own dedicated supply via a relay, it'll work fine. Unfortunately many assume that if they just wire a new horn to the original terminals that were on the old horn, it will work. It might, for a while, but you only need a bad earth and it'll just pack up. The wiring tends to heat up, corrosion set in and ultimately.....no horn.
The relay doesn't need to be anything special - an old headlamp relay would do the job. The original horn drew very little current so didn't need one.
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Thanks for all the help but it turned out that my issue was due to a worn contact under the steering wheel. If you remove the wheel, there's circular contact plate with a little sprung switch in it that is meant to push against a corresponding contact ring underneath the steering wheel. The spring was shot so it's been temporarily bodged with an ice lolly stick whilst i find replacement parts.