You missed the most important part. You MUST have a wire from the - terminal of the battery to the body. This is the 0V reference (and return) for all of the electrical loads on the body. The big one to the engine is only for the starter, coil and other engine mounted solenoids or items that need a 0V reference to work like temperature sensor.
Remember the engine& box is connected to the body via rubber. The exhaust and diff are connected to the body via rubber. About the only body to engine metallic connections are things like choke cable metal sheath or throttle linkage on early cars like HK-HG. Which is why things go haywire without the original 0V connection in place.
Using 0V is technically correct as we both know.
But for the sake of clarity for those with limited electrical knowledge I think we should keep using negative, ground and or earth.
It makes it simpler for those trying to work it all out.
But seriously guys. Ive posted about it quite a few times.
If you are going to play with the wiring, please go buy a multimeter.
It makes life easier for you, and when you need help it makes it easier for the person with knowledge to give you the right advice.
And if given a set of things to measure with the multimeter, do those, nothing else, and sometimes even the order of the tests makes a difference.
Jaycar sell a reasonably priced multimeter that also has a clamp ammeter on it which will measure DC Amps.
Last I looked it was well under a hundred bucks.
It could mean the difference between spending days chasing a problem or just an hour or so.
Cheers
Rob