Does the factory alarm ('76) use a separate horn or the pair of regular horns?  If separate, where is that bad boy located?  Also, there is a little switch that's supposed to be installed adjacent to the key switch inside the fender.  I believe that it serves as a tamper resistant mechanism such that if the key switch is messed with it will activate the alarm.  How do you install that switch?  Should the button be kept pressed by the "L" terminal on the key switch?  Or should it just be very close but not depressed?  

The alarm horn is mounted at the rear of the car, driver side, on the rear side of the wheelwell. It's above the muffler.
The key switch should have a switch(anti-tamper) attached to a bracket, and the plunger for the switch does use the "L" leg of the switch to hold it down. The switch is the same one used on the headlight buckets for the "Headlamps" light in the console.

Thanks, Joel.  I found the horn.  I'm in the process of testing other electrical items (e.g. washer fluid pump, regular horns, etc.).  I thought I'd do some testing on the alarm system and try to get it functional, but now it seems like that could be a lot of time and effort.  That little anti-tamper switch is broken - not enough threads to secure it.  I tested it and it looks like with the button pressed the switch is open which tells me that leaving it out and disconnected should be OK.  But even if I do that, there are a lot of other problematic parts like door and hood switches.  Probably why the previous owner just told me not to use it because it drains the battery.....  Might save this project for another day....

I repaired mine on the 77 I have the electrical circuit for the 77

Thanks, Bob.  I just PM'd ya.  Don't go to a lot of trouble, but if you have some helpful stuff I'd like to see it.  Thanks.