My sons lights all work but headlights.

All lights work but headlights, the park
lights work, blinkers, we checked ground
wires, bought new lights, checked the
relays, have power when plugged to meter
but the second that you plug the light in it
loses all power to the headlights. The park
lights and blinkers work too! New
terminals, checked relays, no problems
with anything else what are we missing??
HELP!