The horns are in same location for all FMJ vehicles. All FMJ’s come standard with one horn and it is an option for the second horns (a Low and High pitch one). I believe the High pitched one comes standard (but might be the Low one, i can't remember).
It is not hard to add the second horn if your car only came with one. There is a short jumper wire (looks like a “Y”) as well as obtaining the other side horn. Using F body Deconstructor Jim’s picture (thank you!), which this appears to be from an ’80 F body, the Horn on driver’s side is the one all FMJ’s come with, sandard (blue circle).
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The horn on passenger side (red circle), the optional one.
The yellow box is the “Y” harness that connects to both horns. The white arrow connector either connects to driver’s side horn or to the “Y” harness. A person can make a harness or even splice right into the green wire as a tap for other horn (be sure to use a good watertight connector) – if needed.
Most Fifth Ave’s come with both horns standard. Everything else “it depends”.
If you notice, the High and Low horns are twisted in different directions and are stamped “HI” or “LO”.
The “non-Road Runner” horn part numbers are:
2808868 – Low
2808869 – High
I haven’t been able to find a part number on the “Y” harness.
The Road Runners have a “Beep Beep” single horn and is, for the most part, different.
Note: for a daily driven car, I do recommend adding the second horn.
For cars with limited use, a single horn should work fine – after all how many times does a person use the horn? (No need to answer that rhetorical question).
Getting back to original question:
There are three main trouble areas for inoperative horns: bad horn, bad horn relay or a bad switch.
I would assume the horn switch is new – so doubtful that is the problem. Missing horns can be a problem. A bad or missing horn relay is also an issue.
I scanned these pictures from my ’86 FSM (Factory Service Manual) which came out as PDF files. Poor quality scans (sorry). These pictures should either match your car or come pretty close.
I can attach the PDF files if you want to see them – but for convenience, I converted them to JPG’s for you and attached.
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The horn contact wire (after it leaves the steering wheel) is part of the turn signal switch (and t/s switch wiring).
The horn relay is in the fuse box.
I hope this helps,
BudW