If you use a non-OEM diagram, watch for:

Finding a reliable is the first—and most critical—step in any serious automotive electrical repair. Unlike generic aftermarket diagrams, the W639 requires a model-specific approach due to its use of CAN-bus networks, SAM (Signal Acquisition Module) control units, and model-year variations.

Located in the fuse box under the hood (driver’s side near the brake booster), this is the brain of the body electrics. On the diagram, you will see components labeled like N10/1f5 (Fuse 5) or N10/1kM (Relay). The SAM controls: