It could be that you have the compressor hooked directly to the battery, without any sort of power switch (such as the horn button on the steering wheel.) Is the air horn after-market? Does your truck have it's factory horn? If so, you may be able to disconnect the original horn (or locate the wires if already disconnected) and run these wires to your air horn's compressor, using the steering wheel horn to operate.
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It could be that you have the compressor hooked directly to the battery, without any sort of power switch (such as the horn button on the steering wheel.) Is the air horn after-market? Does your truck have it's factory horn? If so, you may be able to disconnect the original horn (or locate the wires if already disconnected) and run these wires to your air horn's compressor, using the steering wheel horn to operate.