Well, I think I understand now. It looks like the horn switch gets its
ground from the steering wheel, which is grounded via the steering
shaft.

It *looks* like the steering shaft itself is supposed to be grounded
by the ignition switch, which isn't doing its job. The outside, the
steering column, is grounded perfectly, but the steering shaft is
intermittently grounded, depending on the position of the ignition
switch. I've also noticed that just wiggling the key while it's
inserted in the ignition will cause the ground to come and go.

I just wish I could figure out an alternative to replacing the
ignition switch! I can't even get it off right now, since I'm on a
road trip and have limited tools. (no drill) I tried running a
grounding wire to the steering wheel, which works, except for the
safety issue of having a wire attached to a steering wheel, getting
tangled, etc.

Does anyone know of a way I could ground the *inside* of the steering
shaft? The rack end is sealed and lubricated on the inside, right? So
the only way/place I could do it would be the exposed end at the
wheel, or where the ignition locks the shaft, right?

Thanks for any suggestions!