I bet you they had an auxiliary air horn... Someone already said it in here, but when I first saw the photo, I thought the same thing.
I was thinking of putting a dukes of Hazzard horn or train horn on my 72 Chevy truck and didn't pull the trigger, but when I was looking at them at the auto store (years ago), that's what the compressors looked like. And I bet they just did a good wiring-concealment job and bunched it with the power wires to the brake/towing/accessory bunch of wires.
Now... I may rethink it and might put a train horn in my truck now. Or, my Jeep and if I ever sell it, I'll make sure to tell the next care-taker, so they're not confused as hell.