I restored my grandfather's 8N several years ago. Wanting to get the headlights working. They will work if I run a separate ground wire to the tractor chassis. But, won't work without the ground wire. Is this a matter of too much paint? I'm thinking that the paint is not allowing the light to ground?

I avoided the issue by pulling two wires into each light housing so I could run ground wires back to where the battery ground strap is bolted. Seems like I had to use fixture wire that had thinner high temp insulation to fit both of them through the hole in the mounting bolt.

paint and/or rust and corrosion. There are several sheet metal, battery box, and other connections that current has to go through to get back to the battery ground connection. As advised run the auxiliary ground wire and have a reliable fix for the problem

Not uncommon.

I always run a dedicated ground wire to the lamp mounting bolts and terminate with a suitable terminal with star washer.

Never any problems.