That red wire has to be someones fixit. put a multimeter on the green wire and then see if the volts go up when you push the horn button (assuming the horn button on your truck works). That is the correct way. If it were me I would just hook the green wire to the horn and push the button. 
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1972 C20 Suburban- Big Blue Betty
'56 Chevy Bel Air Sedan- Frame up Restoration
-What would you attempt to achieve if you knew you could not fail?-
-I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door-
R.I.P. EAST SIDE LOW LIFE
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