You can purchase original factory diagnostic CDs for specific model years (e.g., 2008 M2 106) on eBay. Ensure the disc includes the "Electrical Wiring Diagram" section.
The best source is the system available at authorized dealers. However, Freightliner also offers a subscription service to AccessFreightliner.com . For a small daily fee, you can download the official, VIN-specific wiring diagrams in high-resolution PDF. This is the gold standard. diagrama electrico freightliner m2 pdf