I thought all you had to do was go to Options > Controls: Select Wheel/Joystick (DFGT should be automatically detected).
On Buttons 2, next to horn, click on [none] (under the "wheel buttons") and just press the button you want to act as the horn. The same could apply to rest of the buttons.
If you have the logitech profiler installed, you should be record a keystroke and assign it for a button too, but that's the long way around; but needed if want to map the indicators.