Open this link and scroll down to the picture in post 44 and 45 that shows the back of the 2 different alternators and discription..
http://67-72chevytrucks.com/vboard/s...=415738&page=2
If your alternator is hooked up to a small black box looking thing (regulator) mounted to the radiator support buy the horn its probably external
In this thread post #9 on the bottom pic shoes the wireing diagran and the modification to the regulator plug. Just below the alternator shows the brown and read wire being looped back into the regulator plug.
http://67-72chevytrucks.com/vboard/s...d.php?t=417872
Both these threads will shead a lot of light.