Fuel Gauge drops when lights switch on

Hi All,
Have electrical gremlins
I have a 200TDI 110 1991 and when I switch my lights on the fuel gauge drops to zero, if i switch lights off fuel gauge immediately goes back up !
Its only just started doing it today.
Not sure where to start ? is it a short or bad earth, which end to investigate first rear wiring or dash ?
Anyone else had this type of problem ?
Many thanks.

prob faulty at the back end, i replaced the fuel gauge wiring but you could just swap the fuel light wiring over to the gauge wiring and vice versa

Many thanks will give it a go at the weekend.

i had this but then it started to drop the fuel, temp and voltmeter after a while, turned out to be a corroded fuse in my case and was a 2 minute fix

Cheers Steve