The headlights on/in use signal wire is usually wired [in the vehicle] to the dashboard lights that light up when the headlights are on rather than wired to the headlights proper (if you catch my meaning).

This is mostly for the reason that the wiring is right there - well within 2 feet of the din slots where the tuner/headunits go - simply connect to the wires that light the dashboard up and theres your "headlights on" signal wire.