OLM Hella Horn Harness - 15+ WRX/STI
This simple harness plugs into your factory wiring and gives you a couple sets of outputs you can connect directly to a pair of hella horns. As seen in the picture there are two locations where this can plug in usually. On newer models, you may only be able to plug it into the "F5" connector. Be sure to hold the wires by the base and make sure they are aligned as you connect the male and female connectors. Once you have plugged it in, use the supplied heat shrink over the connection to hold it snug.
Connect one pair of shorter wires to one horn and a pair of longer wires to the other. That's it!