Sorry but the Stebel harness was not one of my products. It should be a very simple harness. Connect the OEM plug to a relay to energize the coil and run a fused line from the battery to one side the Normally Open (NO) contact of the relay and connect the input for the Stebel to the other side of the NO contact. Push the horn button and the relay closes making the horn blow. The tricky bit is making sure the relay is rated for the current needed for the Stebel horn, size the wire and fuse accordingly for the current needed by the horn.
I could build one, but not having a Venture around to determine what connector is needed or estimate the length or wire needed, I would need some help in the form of pictures of the horn connector, and estimate of length of wire needed to get from the battery to the new horn.