Starter engages when I turn the l headlights on

Hello I have a 1987 f250 dieseil lariat.

After charging the battieres last the starter engages when I turn the lheadights on. Futher more. The ingition works fine and the truck starts the way it should but at night when I turn the lights on it engages the starter. I cant turn the lights on while the motor is running or the starter grinds aganist the flywheel or something.

Thanks for the help :)