I need help 2013 v6 Ford Mustang
 

I am stumped on what can be wrong with my wife’s car, we replaced the Alternator, the fuel pump and the ignition switch and the battery. Her car now runs but when we turn on the headlights her car stalls and drains the battery.
One day she had it running and had the headlights on, air conditioner, stereo and blinkers and she never stalled but soon as we closed the hood thinking we had it fixed she stalled.
please help, we have taken it to 3 mechanics and replaced parts that didn’t need to be replaced