That the gauges change readings when you turn on the lights directly indicates a ground path failure .

Remove the gauge binnacle and take the time to ascertain the ground paths, at least one will have a screw or other fastener in it, use this to add a brown wire from the binnacle proper to a good ground point on the body .

Once you've done this and have the binnacle re installed, remove the tank sender using s 46MM socket of large pair of Water Pump Pliers, turn the key on , re connect the sender and invert it whilst you watch the dash gauge .

It should go from empty to full, kif not you'll likely have to carefully and gently take the fuel sender apart and clean it well using soft brushes like paint brushes ~ it's very easy to break the two delicate fuel sender wires , I am sure there's plenty of threads concerning sender cleaning ~ I've saved almost every one I took apart .

Keep us posted and try to remember : you're not a Mechanic so it may take you a bit more time and effort but you will get there .

I can dig your car being a "B.I.T.S.A", I used to build them, and Motos too, you're cleaning up some one else's lazy / sloppy job to don't let anyone rile you .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better