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 :)