I would think the horn is unrelated, the wiring for it doesn't run with the combi switch. Have you checked if you may have damaged the wiring in the column while running the new combi wiring?

__________________
Alex
MB Star certified, now independent.
Owner/Operator FDMotorsport Mpls. MN
Mercedes and GM certified transmission rebuilder
We all need a little help sometimes...

1998 E320 4MATIC-- stolen by my sister
1989 300TE-- sold
1999 C230 Kompressor