Never thought a Horn could be so challenging

[DetroitIron]

After hours of researching this site and others why the horn on my 85 K20 doesn't work, I wondered if some experts might be able to narrow it down.

Here's what I've done/tried:
1) The horns work when running a jumper wire from the battery
2) The horn fuse is good and I'm getting 12 volts measured with a multimeter in the horn fuse location under the dash
3) My horn relay clicks (does that mean it's for sure ok?) when I push the horn button
4) Pulled my wire harness bulkhead connector and checked continuity of the horn wires running from firewall to horns. All good there.

Any ideas? Thanks for any help you can offer.