An old rim-blow horn, eh?
Tilt column as well?
If it's a tilt column, chances are it'll be a Saginaw column sourced through GM (Chrysler didn't make tilt columns back then) and will have an adapter harness between the Saginaw turn signal switch pigtail and the Imperial body harness.
Probe the black wire at one end of the turn signal plug and see if it grounds when you squeeze the horn ring. If it doesn't, jump the black wire to ground. If the horn blows, the circuitry is ok but the rim-blow horn probably has an open circuit and you'll need to replace the steering wheel itself. If you pop off the center pad and short the horn wire to ground and the horn blows, the steering wheel is the culprit.
I ran into this issue in 1989 on an Imperial owned by Tex Smith (Hot Rod Mechanix, Custom Car, Rod Action Magazine, Hot Rod Magazine, Rod & Custom, etc). He couldn't get a state inspection on the car because the horn didn't work so I fixed it with a Radio Shack push button and he sold me the '72 Newport Custom he had bought so that Pegge could have a car to drive.
Somebody had disconnected the cruise control button and re-wired it into a plastic horn relay out on the left fender brace and, being out in the elements, it corroded and the horn quit. The plastic relay was designed to be mounted INSIDE the car on the fuse box, not out under the hood where it was subject to water intrusion.
I reconnected the cruise control and got that working again then Tex and I drove over to the local Radio Shack, bought a tiny momentary push button switch, and placed it dead center in the steering wheel pad. I could have put the button elsewhere but it was decided to put it where it should be most logically located were it be need in a hurry. It ruined the cross piece but that wasn't my decision.
The metal factory horn relay was still installed so as soon as I grounded the horn wire with the Radio Shack switch, the horn worked.
When I finished, everything was factory correct except for the added switch. Pegge was ecstatic that she had her Imperial running again and WITH cruise control to boot.
Saginaw rim-blow horns were prone to failure back then. If you have a flat, GM-style turn signal plug on your steering column, it's a Saginaw column, not a MoPar column.
BTW, that beige Newport in my avitar is the very same Newport I bought from Tex out in Idaho in '89.