compustar into lexus sc

Often times the horn wire is in the yellow colored wire looming that also carries the wiring to the SRS Airbag -- be VERY careful with this loom  as if you mess with the wrong wire, you can risk deploying your airbag (never had this happen to me, knock on wood)

Purchase a multimeter (aka Voltmeter, VMM, MM) and test the horn wire once you find it by activating the horn and noticing the voltage change in the wire you're testing.