Here is engine OEM parts diagram for 2009 Lexus RX 350. This engine parts diagram 2009 Lexus RX 350 is for 3.5L V6 Engine.
Lexus electrical schematic
Here Lexus typically air horn relay wiring diagram. After you bought all part required, this wiring diagram will help you about horn installation. Hope match with your requirement, because it all work in same way. Take the original headlight + wire and run that to terminal 85, then you take +12v from the battery (make sure you install a fuse, 25 amp is sufficient, 30 will work), and run that to terminal 30. Terminal 86 needs to run to ground, does not need to be high amperage, terminal 87 is the positive wire that you run to the light themselves (ground for the headlights is not run through a relay and can be any standard chassis ground).