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What year is the highlander? What is the amperage that the air horn is calling for (how much current draw) is required to drive the pump an solenoid?
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You are going to have to wire this independent of the horn relay in the car now. Click the link below and save for a picture. This is only a 10 amp horn fuse and system. Do not attempt to use the black wires going to the horns to drive pump and after market horns. This system uses solid state relays and they are built in to the fuse box under the hood and are not serviceable separately. You will damage the fuse block! You can take the signal from the solid state factory relay (black wire) to energize the relay that you have to supply power from the battery through those contacts. This will make the load for you after market horns go through the relay it came with. The grill is part of the bumper cover and the bumper cover. the pop clips have a center button that pushes in then the clip assembly pulls out of the hole to get the top plastic shields off.