This is something that I have experienced with older cars but not something this new. My old WRX would dim the headlights when I hit the brakes. Don't get me started on my old Blazers.
If you have the dash lights up (shouldn't the lights be out if the headlights are out?) radio on and the defrosters/AC on (the condenser running) when you hit the brakes its a definite draw on the electrical system.
What all do you have drawing power? Mods, etc...?
If you've come to help me, you're wasting your time.