sick truck

i have a 1988 s-15 gmc reg cab 4.3 no matter where the fuel gauge sits u turn on head lights or park lights the fuel gauge drop at least a 1/4 on the gauge turn off lights off it comes back up any help please

i have a 1988 s-15 gmc reg cab 4.3 no matter where the fuel gauge sits u turn on head lights or park lights the fuel gauge drop at least a 1/4 on the gauge turn off lights off it comes back up any help please
What all is sick on it just gauge or other?
How is fuel pressure and ign system.
Any codes or check engine lights?
Have you checked grounds on fuel tank and other electrical stuff.
Is alternator carrying head light load and other stuff?
What is battery voltage reading engine off andf on and with all electrical stuff on?
Does any other gauge move with headlights on?