My brother has a 2000 Toyota Tacoma we cant figure out why the heck is it doing this. When you turn on the headlights on the truck dies and the starter starts engauging and it wont start again until ypu turn the headlights off got us baffald the alternator tested good

 

You could throw parts at it and maybe get lucky or you could get a wiring diagram and use a multi-meter to diagnose the problem.
I would start with checking all your grounds and inspect the ECM power feed as well as all of the connectors in the steering column for headlights and ignition switch for loose terminals or burnt wires.
Check the headlight switch and ignition switch for proper operation.
If you have a remote starter system this could easily be a problem with those connections since those aftermarket systems tap into the starter and running lights.
check all of the wires under the driver's side of the dash and in the steering column for signs of accessories tapped into the stock wiring.

 

how did you guys solve that ?