If you think you must have a ground wire to the tail light to make it work you are wrong.
As is virtually every vehicle maker - except British bike makers - since WW2 (some even earlier)? I'm afraid it is you who is wrong, as you would know if you'd looked at the electrics of any other vehicle in the last 40 years except your '72 T120.
1. The basic principle of vehicle electrics is that, once the electrons leave battery -ve, until they arrive back at battery +ve, a given electrical component does not 'work'.
2. As most vehicles are at least mostly made of metal, at least one half of any electrical circuit - battery to component or vice versa
- must be insulated from the rest of the vehicle; ime, this is done in the aforementioned proper, purpose-made electrical cable.
3. If you connect one side of the battery to a part of a mostly-metal vehicle and, by simply bolting an electrical component somewhere else on the vehicle, you anticipate that component will 'work', all
you're doing is expecting the parts of the vehicle between the component and the battery to form the other half of the component's circuit.
4. In the electrics we're talking about, electrons cannot move through the paint, powder-coat, oxide, etc. that tend to be present on motorcycle parts, so any parts that have one or more of those things on its surface cannot, by definition, form part of an electrical circuit.
5. It is pointless is to scrape the paint, powder-coat, oxide, etc. off the metal because it will simply oxidise again where it's exposed to air.
It may not be a perfect 12.5 volts
So, given all the above, why would you not
have both sides of any vehicle electrical circuit in proper, purpose-made electrical cable?
As did and does the aforementioned "virtually every vehicle maker - except British bike makers - since WW2 (some even earlier)"? Even Lucas
on harnesses it made for British cars long before it started fitting the basic Red cables' network in bike harnesses?
The 72 positive ground bikes had a separate harness just for the tail light.
As do '71, '73, '74, '75, etc., etc. It isn't peculiar to '72.
In my previous post is, "In a standard '72 Triumph (or BSA) harness, the network of Red (earth/ground) cables is connected directly to battery +ve. Therefore, for any given electrical circuit, both the supply from the battery (and rectifier) to the component and the return from the component to the battery are in proper, purpose-made electrical cables". At the risk of labouring the point, those are the specifics for a '72 T120 in the context of the generalities above.
To continue to be specific - Lucas
supplied the three-wire rear lamp harness, the main harness and the rear lamp; Lucas
intended that the three wires in the rear lamp harness should be connected at one end to the three correspondingly-coloured wires in the rear lamp and, at the other end, to three correspondingly-coloured wires in the main harness. If you do that, both the tail lamp and brake lamp circuits are completely in "proper, purpose-made electrical cables".
If you read the initial post,
He also states he has tail light with 2 wires,
I have seen these,
I have too. As I knew as soon as I read the original post, and as Les explained clearly, one wire would be connected to the tail lamp filament and the other would be connected to the stop lamp filament. For the record, this is exactly how your bike's standard '72 Lucas
"tail light" was before '71.
or you have to bring a ground wire to it and connect it to the metal base of the bulb holder.
And why would Mark not do that?
Especially as his bike has the end of a Red "ground" wire a few inches away? In the experience of everyone who's done it, or owned a vehicle so wired, that makes for reliable tail and stop lamps, unaffected by the dampness of next winter or the one after that.
If the battery is grounded to the frame via a wire, the entire motorcycle is grounded.
You're wrong. Using your terminology, the only
parts of the motorcycle "grounded" are those in metal-to-metal contact with the wire to the battery. As I've said, paint, powder-coat, metal oxide, yadda, yadda are not
"metal-to-metal contact". Worst case, if the person who connected the wire from the battery to the frame failed to remove the paint or powder-coating at the frame contact point, not one single part of "the entire motorcycle is grounded".
While we're here, you are also confused about "ground". On any vehicle, whether it has a network of cables for the purpose or the maker has used other vehicle parts for one side of the electrical circuits, that's all it is - one side of the electrical circuits. What it is not
I purchased my 72 TR6R new in 1972 so I have owned if for 40 years. I have rebuilt and rewired it several times.
And I've rewired probably 40 Triumphs and BSA's - never mind other makes of bikes - in the last thirty years. I've never had to rewire any bike more than once.