I went to fix the horn in my 1975 Spider a couple weeks ago and found there was no compressor. I looked around and found a horn kit at Harbor Freight that was on sale for about $13.
Here's the link. http://www.harborfreight.com/12-volt-su ... 40134.html
Tonight I finally got some time to work on the car and got it hooked up. The kit came with everything but I just used the new compressor with the original trumpets. On the 75 the wire that runs to the horn is yellow so I hooked it to the positive terminal, put a new wire on to ground to the chassis mount, and then hooked the air hose up. I also popped the button off the steering wheel and cleaned it with sandpaper. After that, it worked great! Check out the link for the Youtube video of the original Italian sound.
https://www.youtube.com/watch?v=lU-vVAUH2q4