Dedicated chassis ground, usually a Black wire.
The is not just a map of wires; it is your guide to understanding the logic of the HVAC system. For manual systems, remember the triangle: Fuse → Motor (+) → Motor (-) → Resistor → Switch → Ground . For digital systems, respect the LIN bus data line and the fragile control module. Freightliner M2 Blower Motor Wiring Diagram