The minus is connected to ground (Audi A4 B5)

The current is supplied to the consumer via a wire, which can always be traced back to the battery in a more or less complex way, as can be seen in the electrical diagrams in this chapter.

Feedback is provided by conductive metal parts of the body or engine. This saves a large amount of copper wires. In some cases, however, a short piece of wire is needed to provide feedback, especially in cases where the consumer is not placed directly on the metal. These wires, connected to the ground, always lead to one of the so-called ground points, which are scattered around the body and to which the eyelets for attaching the wires are screwed.

The negative pole of the battery, of course, is also connected to the body - "ground", as an auto electrician would say. Remember: "minus to ground".

[This publication is borrowed from the resource «AudiManual.ru»]