Not sure if the horn contacts switch power or ground on that vehicle But a schematic or at least knowing how the horn circuit works would be a good place to start as that affects the issue.
Silly thought, have you tried disconnecting the actual horns to see if that affects the problem. If so by chance, try hooking one at a time up and see if the issue returns. Might also listen for a relay clicking, locate it and do the same. Horns typically draw a reasonable amount of current and if there's a flakey ground somewhere it might be backtracking through the steering wheel contact and WHACKING the cluster or PCM