Hi Ducati
A quick lesson on car electrics.
The colored wires to any item on the car carry + current to the item (radio, lights, whatever) Different colors and stripes to keep identity separate.
In order for the current to flow through the item and make it work, it needs to have a return path to the - terminal on the battery. In other words it needs to complete the circuit.
If this circuit does not get completed, the item
will not work .
The way car makers and Bike makers complete this circuit (commonly called a "ground") to the - terminal on the battery, is by using the steel body of the car as a "wire", instead of running all these wires back to the - terminal. German cars usually use a brown color for the ground wire, and the rest of the world uses black. Sometimes you will see a neg (-) wire coming off of an item which gets screwed into the body somewhere else, or the item will use a mounting screw as a ground.
Anyway, I hope you can use this tutorial and if you know this stuff already, then good
If not, then good
I have had luck with teaching people these basics with this tutorial of mine, and it helped them over this hurdle.