How do you pass the wires for a horn from the engine compartment to the inside of the car?

When confronted with this dilemma (has happened many times) I look inside the egine compartment, along the firewall, for a rubber grommet which covers a hole, in the firewall, through which wires or cables have been routed, by the manufacturer. It's crude; but I loosen the grommet and verify that it cleanly accesses the passenger compartment, then I drill or cut a small hole, through the grommet, run the wires through, reposition the grommet, and, if necessary, put some rubberized sealant over the newly drilled or cut hole, to prevent water leaks into the passenger compartment. Works fine for me.