Don't waste any more time looking for the wire at a 5 wire relay isolator....it doesn't exist!
The horn switch in the steering wheel is strictly a 'Mux' circuit that is read by the steering control module and sent via data to the instrument cluster where it travels via the Can Bus to the TIPM. Basically, there is nothing to 'tap' into.
Once you get to the TIPM, there are options. Connector C3 on the bottom (C), Pin 21 with a Gray/Light Blue wire supplies power for Horn #1, and Pin 25 with a Gray/Yellow wire supplies power for Horn #2. The other side of both horns is tied to ground, so either of these wires could be wired through a relay to be use the horn for the remote start. Having said that, why does the remote start need a connection to the horn ???