where does this connect? 71 challenger . about 3 ft long black wire in photo . comes out of the harness at the bottom. about 3' long.

Damn, I can't recall any wire with an eyelet terminal aside the alternator post wire, but is 12 gauge and that's not 12 gauge. New harness?

Need a factory service manual, which I don't have for that, sorry.
That length & terminal suggests a coil connection or early NSS...

is that the engine harness connector? if so pin 20?
IF so it shows nothing there on a regular schematic. So my guess would be for an accessory of some sort (fog lamp/ convertible top relay) or possible emissions / tach ? What color is on the other side if you can see it.

I wondered about a tach pickup connection ? which might make sense that it's not showing on the schematic I looked at as it was not standard on all models. Also thought about the early pre 65? NSS on the automatics but not in a 71 harness.
To The OP, go back to your original harness again

nothing in that one . looks like it may have had somthing in it. no sure. i will check the back side but it will be later. thanks !!!!

tach with that huge eyelet terminal ? I don't think so.

Do you have a service manual for 71. The wiring charts would help clear up many of your questions. Certainly the E body site would be more helpful considering the level you trying to take the car too. Just a thought.

Reminds me of a ground wire.

Or coil, save for the terminal doesn't have provision to clock it? 3' from where it's at would get it over there?

Nacho I'm not one to usually question you but HUGE ? looks like a size 8-10 ring terminal to me I believe the coil terminals are # 8 or maybe 10? Wire size 16?

Silly question Where did the harness come from? Any chance of contacting them and asking

3/16ths hole ..... the old harness has nothing in it. the new one has the black wire outside and inside looks like white/pink stripe... i hate wiring. i have hi/lo headlights,turn sigs,brake lights,emerg flashers,radio ,clock starters working heat/fan .... no-parking/tail lights ,side markers,or tag .. dash or dome tag or reverse. going to put some fuel in and see if i have spark!

Have you called whoever made the replacement harness ?
Is the interior side connected to anything ?
That color coding seems pretty weird, white/pink to black...
Could be an add-on...

Is this "WIRE" part of the engine, light or wiper harness?

3/16ths hole size is for a size 12 fastener which is a bit uncommon Can you call the supplier or manufacturer?
If it's part of the engine harness A factory service manual would answer the question.
If it's the light or wiper harness that schematic is available for free at My Mopar.net in the body manual

FSM shows a grey coming from cavity 17 to the oil pressure switch. Any chance it's dark grey?
There's also a grey from cavity 20 to the coil.
Black wires come out of 7 and 8 for the back up lamp switch.
A black wire comes out of cavity 12 to the brake warning switch.
A black wire comes out of 18 and goes to the alternator.

Zooming in, it looks like it's coming out of cavity 20 and should be a grey wire with a tracer. Cavity 20 is next to cavity 24. It should be, in line, grey with tracer to coil negative (cavity 20), brown to windshield washer motor (cavity 19), black to alternator (cavity 18), and grey to oil pressure switch (cavity 17).
Next to those should be dark green with red tracer to horns (cavity 24), dark blue with a tracer to a splice that then connects to voltage regulator/alternator field/ballast (cavity 23), brown to ballast (cavity 22), and violet to the temperature sender (cavity 21).

A shot from the '71 FSM

it is position 20.... should be a gray wire and it is for the tach. the repo harness came with the black wire. i will swap the factory gray wire with the black. i connected the black wire and it worked the tach...thanks !

Be careful about using some of the free wiring diagrams online. I've found a few errors in the '71 diagrams on mymopar.com

FINALLY I'M NOT THE ONLY ONE MAKING THIS WARNING CALL!

Ppl don't stop to give this advice to check those diagrams, and even is true they can be helpfull, is not less true it must be taken with care!

thanks for the follow up