We just posted a new application note showing how to connect the ISIS MASTERCELL to any GM-style steering column.
This includes wiring for the horn, turn-signals and 4-way flashers. This schematic also shows how to wire in the turn-signals so that they only operate when the ignition is on. You can get the PDF of this application note at this link.
There are many other application notes that will help you wire your car with the ISIS Intelligent Multiplex System. Check them out in our reference library.