Negative - (Black) is Ground, Positive + (Red) is Hot. Ground wire goes to Negative (Black) it goes from negative on battery to engine for ground.
Mostly you will see it bolted to the engine.
If you mean your gauges are going crazy, this is likely the result of a loose or bad ground wire. It could be the ground wire for your battery or any ground wire that is tied into the gauges.
that would be the negative normally in most cars goes to ground on the engine block
to the frame of car
its either the alternator the battery is bad or a loose or bad wire on the starter
check the ground for the battery, sounds like the car is grounding itself out right away. even if the ground feels like it is attached solid it could still have enough corrosion inside the wire to cause it to ground out.
I assume the wire in question is the smaller blue wire. Yes it travels inside the car to the central electrics (fuse block) then to the instrument cluster. The cluster feed the battery light power and finds a ground through said wire at the alternator. when the alt starts charging, there is power on that wire from the altwhich doesn't let the bulb get ground and the light is out.
it goes from the starter mounting bolt to the battery box.
you most likely have a loose or unattached ground wire to some major component.
The thick black wire is the negative which should be connected to the frame of the ATV (negative ground). The red wire comes from the battery positive terminal and should go to the start solenoid From the other side of start solenoid a red wire of the same size as the black one should go to the starter.
If the lights are going off and the car dies at the same time it sounds like a bad ground. Check your battery post terminals. Make sure they are tight. Also check where the ground wire from the battery mounts to the engine. Check the positive cable at the starter. Make sure it's tight.
Typically a poor ground, or some how the red wire is touching the black