2002 f-150 No start with headlights on or brake pedal depressed

2002 ford f-150 5.4l. The engine will start and run but as soon as you turn on the headlights or press the brake pedal the engine will stall. The DLC is also dead. This truck was in a front end accident and the airbag was deployed. I have not really tried to diagnose this yet but its waiting for me. Does anybody know what the headlight and brake light circuits have in common.

I'll take a shot in the dark and offer a scenario.

If the Tail-lights and the Brake lights share a common ground, and that ground is bad (but the splice is good), current could back-flow from one, to the other.

i.e. Current flows into a bulb's power, through the filament, out the bulb's ground, and though a ground splice. One leg of the splice is the actual chassis/battery ground. If the actual ground is bad, but the splice is good, current will back flow from one bulb to another.

Current back-flowing through a ground will play havoc with anything trying to use that ground.

If a vital component uses that ground, it may have a "good enough" ground to work, if nothing else is loading the ground, but when another component loads the ground, the voltage on the ground may become so high that the vital component no longer has a great enough difference in potential, and stops working.

The first thing you should do is find out what is missing when it does not start.

Turn the headlights on and while attempting to start check for spark, injector pulse, fuel pressure... what's missing?

I add a link to one of my favorite u-tube videos to illustrate what Chad already mentioned:

YOU most likly have a short to gound . look at all wiring on your air bag senors on bumper short on wiring to head lights , because of your wreck . GOOD LUCK