This photo of the relay rail and wiring cluster shows the circuit where the horn feed starts. For more info see the Vanagon wiring diagram circuit #53.
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Detail of the horn circuit feed point, which can be a little difficult to find:
Output from fuse #12 connector, extreme right side in the photo, feeds the horn. By default this is served by an 8 amp fuse, which will need to be boosted for 2 horns. Each horn pulls about 5.2 amps, I measured it. This is a black/yellow stripe 18 gauge running down to the horn.
This terminal will be replaced with the power jumper into the relay at socket 25. Remove it and bundle it together with the brown wire in the next step.

A brown 18 gauge wire runs back up from the horn through a white connector left of the fuse cluster as shown in the photo. A brown/blue wire runs from that connector to the horn contact ring. Which in old Vanagons is suboptimal for the 5.2 amps needed for properly grounding the horn. Stuff wears out.
The horn relay looked like it would fit in the 2nd relay socket from the left with a little rewiring. This old '83 diesel Vanagon was missing the seat belt warning relay otherwise meant to be in there, so I decided to use that socket. But there would be nothing wrong with the relay floating around in there loosely if that relay socket were not available.

Four connectors need to be operative. Starting with control in (black) at #27 and control out at #21 which goes off to the steering wheel (horizontal in this photo). The seat belt warning relay was powered from the #11 fuse into socket 27, so I left that wire as-is. This however means two fuses are needed to power the horn system, just to keep it in mind in case of horn failure. You could optionally power the control circuit from the spare output from fuse 12.
To use this relay socket, you will need to remove four clips:
Extract the grey/black wire from socket #25 and insulate it, this terminal is live while a key is in the ignition, so watch out!
Remove the clip from socket #24, this yellow/red wire powers up the seat belt warning light. I attached the grey/black wire to this wire so the dashboard light glows when I have forgotten the key in the ignition.
Extract the brown double wire from socket #23 and let it float free. It is a ground and needed for another relay so don't cut it off.
Extract the grey double clip from socket #21 and let it float. These wires are grounded when the doors are open.
To remove the existing clips, I used a pointed thin awl-like tool inserting it downward from the top into the slot pushing far enough in to release the clip barb by pushing it inward. A tiny screwdriver would probably also work.

For control out, cut the plastic sleeve from around the brown/blue wire and replace socket #21 with that clip. I used the same tool to bend the female end I was now inserting to cause it to be snapped in unidirectionally. The clips have two horizontal slots which can be pried apart at an angle thus gripping the inside of the socket and locking it in place. This is somewhat of an art, since mutilating the metal too much can prevent a smooth installation of the relay.
Now you can insert the relay, turn on the key and press the horn, you should hear and feel the relay engage. The control circuit is finished.