It is most likely the horn ring contact. There is a copper strip with a contact
pad that rubs the back of the steering wheel that is the electrical contact for the horn. The two of them wear down, and you get the most annoying squeal, exactly like brake pad squeal.
The fix is tricky to make it long lasting. Polish both the contact pad on the end of the metal leaf, and the ring on the back of the steering wheel. A dremel with a felt bob and some polishing compound are very useful here. Both must SHINE like a mirrow or the squeak will come back shortly.
You can also just lubricate it with some heavy lube, but that will wear off.
It may also be the bearing at the end of the steering column, but not likely. If the squeak is at one particular spot, and its right about where you drive straight ahead all day, its that darn horn contact....