a lamp circuit is the second most basic circuiot in existance. A shunt or short IS the most basic.
a shunt / short it pretty much a wire from bat post to bat post.
A lamp is only a lil more technical. it's a wire from hot to a lamp to ground. sure. throw in a switcha nd a fuse. those are invisible to the circuit for all intents and purposes until you open the circuit.
use a test lamp and check to see if you have power to the wires feeding the lamps.
clip the test lamp clip to chassis.. then probe before and after the switch. might be bad connections.
rusty 60 year old tin is NEVER a good ground. I always run a seperate clean chassis ground to my headlamp buckets.