Install something like this. It is pretty simple and does not change the apearance of the auto interior. You will, of course, have to find the right wires and a suitable button in your vehicle.
What this guy has done is find a button to provide a ground path that does not do anything when in the start position. The key is that the defrost button causes a relay to close if the ignition is in the run position. The button is provides a signal that is ignored when the ignition is in the “start” position. With other vehicles you might be able to use a window switch, for example.