Forgot to mention the spring clips that hold the horn push in place form the electrical contact through steering wheel and it's nut to give the switched connection to chassis/ground. Unlikely there would be an interruption there but worth bearing in mind.
I wouldn't pay too much attention to the resistance you measured with a high impedance meter. The horn push contacts are, to a degree, self cleaning with the higher current draw from horn(s). Working the contact by repeated use of horn push may improve matters.
