Wiring headlights for an NAA

If you have a pair of original lights, you will find a ground wire that attaches to the inside of the headlight bucket. As David said, this was probably adequate when the tractor rolled off the assembly line 50 years ago, but there's probably been too much rust and paint jobs in the way for that to be an adequate ground now. I tried using that as a ground for starters and would have to jiggle the right light to get it to work because of the bad ground path. SO I took the lights off again and ran a ground wire back out. This is before I read the advice to run the ground wires back to the battery ground bolt. Instead I ran them to the back side of the front emblem and used one of the emblem mounting screws as a ground. I sanded the paint off the area first so it would be bare metal. Works great so far!

I still need to cut my wires and put in disconnect plugs...that's a real good idea. Otherwise to remove the hood you either have to cut the wires or remove your headlight lamps.

Originally, the headlights were grounded to the frame and there was a
single wire running from the left headlight, over the radiator and
back along the right side of the hood where it was joined by a single
wire from the right headlight.

Most people install a "quick disconnect" (male and female connectors)
in the wire there, so you can remove or raise the hood without having
to cut the wires. But I digress. That wire runs to the switch, which
contains a 20A fuse, and the other side of the switch connects to
*either* the always-hot terminal on the terminal block or the terminal
that's hot only when the ignition switch is on.

(People here have offered differing opinions on which choice is best,
and I don't know which was "original." The advantage of wiring to the
not-always-hot terminal is that you can't leave the lights on
accidentally (unless you also leave the ignition switch on
accidentally); the disadvantage is -- some have said -- the ignition
switch might not last as long if it has to handle the additional
current load caused by the headlights. I played it safe and wired my
light switch to the always-hot terminal.)

If you have rear lights (running light and/or work/utility light),
they worked the same way -- grounded to the frame with a single wire
down inside the fender, along the rear axle (inside some sort of cloth
sheath, originally; often inside a plastic sheath or fuel hose these
days), over the rear end and under the draft control spring,along the
side of the hydraulic and transmission housings, and from there to the
switch.

Now, regarding the grounding: Since I was rewiring the whole thing, I
took Dell's and others' good advice and ran a separate pair of ground
wires as well, because the chassis ground is often not as good today
as it was 50 years ago. I ran them parallel to the headlight and
taillight wires, and grounded them to the battery box/battery ground
terminal.

There's an 6V wiring diagram at the "Vintage Tractor Resource"
that you might find useful; there's lots of other good NAA info at
that site as well.

Good luck, and if any of my rambling above isn't clear or doesn't
match what you've got, don't hesitate to ask more.