Help Troubleshooting Horn Circuit?

My neighbor's 1988 Chevy Caprice's horn stopped working. No other circuits appear to be affected, and there's no fuse just for the horn.

I pulled the lead off the horn and applied 12V straight from the battery to the horn's terminal. The horn sounded normally.

After reattaching the horn wire I pulled the horn relay out from beside the fuse box under the left side of the dash and sent him to AutoZone to buy a new one. Nine dollars later, he was back with it. I plugged it in but still no horn when the buttons are pushed.

I then pulled the relay back out and measured each of the 3 flat connections it plugs in to:

The bottom one had a steady 12V.

One of the others has continuity to frame ground when checked with an ohm meter.

The third lead has no voltage OR a ground.

Pushing the horn buttons on the steering wheel while measuring the 3 relay contacts does not change their respective readings at all. It appears the relay is not receiving the input (ground?) from the steering wheel.

I'm thinking the problem is indeed in the steering wheel, and will cost the guy big bucks to repair. Am I missing anything here?

Yeah, The fact that the repair could be a simple replacement of the horn button with a cheap switch. The horn button in the steering wheel grounds the horn relay to make it work.

The side you have 12 volt on is the feed. One other lead will run to the horn. The last goes to the switch and grounds when you push the horn button.

The problem could be the wire from the wheel contact to the relay , but the more common problem is that the wire under the horn buttons loses contact. Pull the pad off and see if grounding the contact turns the horn on. If it does the wiring to the relay is OK and you just need to repair the wiring in the pad. Not real hard and becomes self explanatory when you open it up.