The following happened to my 95 C1500 5.7L truck, please help me figure this out:

Truck sat up a while in the garage for repairs and when I went to start the truck I connected the battery cable got in the cab and the all the instrument cluster lights were on, the check engine light, low battery light, brake anti lock light, air bag light, all were on like the key was on an the engine had stalled.

Well the ignition switch was in the off position and the keys were in my hands. Inserted the keys into the ignition switch turned it to the run and start positions and back to the off position, instrument cluster lights still on. Repeated turning the ignition switch on and off a few times, same result, lights remained on like the ignition switch was in the run position.

Research led me to the conclusion that the ignition switch went bad, while truck sat idle?

I pulled, inspected, cleaned and reinstalled the ignition key cylinder. (without removing the steering wheel) and there was no apparent defects in the cylinder or the housing for the cylinder all look good a little wear on the parts but appear to be in good shape. Reinstalled the cylinder.

Removed, inspected, and cleaned instrument cluster and interior of dash panel. Reinstalled instrument cluster, and still not right.

I started pulled fuzes and when I removed the Horn fuze the instrument cluster lights went off.

The truck operates correctly except the horn.

How would the horn circuit interfere with the instrument cluster/gauges?

Thanks