On Aug 20 Mark Drillock wrote about upgrading the Vanagon horn.  
He mentioned a Hella horn (for Mercedes Benz models) that is far more 
powerful than the stock Vanagon horn.  Jay Brown (thevwbusguy) then 
wrote mentioning his horn relay -
https://sites.google.com/site/vanagonheadlightrelays/horn-relay

I�ve installed them both, and they�ve got to be the least
expensive safety upgrade 
(at $35 total) that I�ll ever make on my  '87 Westy, which
now speaks with real authority!
Richard Adams 

> Date: Tue, 20 Aug 2013 17:35:25 -0700
> From: mdrillock@COX.NET
> Subject: Horns, improvements
> To: vanagon@GERRY.VANAGON.COM
> 
> The stock Vanagon horn is not that great but can be improved. Simply
> adding a relay down by the horn can make a big difference even with the
> stock horn. Doing that increases the voltage at the horn as well as
> triggers full volume from the horn even when the steering wheel contact
> ring or button have only a weak connection. By using the steering wheel
> button to simply trigger the relay the button can work with only a small
> fraction of the power going through it that the horn itself would need.
> Thus the relay can trigger and remain on with a much weaker connection
> than the horn itself requires for sounding. With the relay providing the
> ground the horn is louder, faster, and more consistent.
> 
> Here is a photo of a horn with the added relay and wiring to increase
> horn performance. The 2 existing horn wires plug into this setup. No
> additional wires need to be run. The yellow end is a male spade. The 2
> existing horn wires have female ends. The stock black/yellow wire plugs
> into the wire with a yellow end. The stock brownish wire plugs into the
> remaining relay male spade pin #86. The wire with the ring terminal gets
> grounded at the mounting bolt for the horn spring plates.
> 
> http://images.thesamba.com/vw/gallery/pix/1101215.jpg
> 
> 
> The horn in that first photo is actually a larger version of the stock
> horn but mounts and wires up the same as a stock horn and in the stock
> location. Here are both horns for a comparison.
> 
> http://images.thesamba.com/vw/gallery/pix/1101214.jpg
> 
> I purchased that larger Hella horn from this place. I installed it in my
> 89 with the added relay. It is now much louder but still has the
> familiar sounding beep known to old VWs.
> 
> http://www.ebay.com/itm/HORN-DISC-12V-335HZ-MERCEDES-/280423157106?
> 
> 
> Of course there are many still louder horns and I like some of the dual
> horn setups from various Volvo, Mercedes, BMW, etc I get at the
> junkyard. These have a totally different sound of authority but are more
> complicated to mount in the stock location. I might get into that at
> another time.
> 
> Mark