Avoid generic "free diagram" websites. Many show outdated diagrams for the FL70 or Century Class, which are NOT the same as the M2's multiplexed or resistor-based system.
If your truck has the digital climate control panel (with an LCD screen and push buttons), the wiring is different. There is . Freightliner M2 Blower Motor Wiring Diagram
If you have the wiring diagram in hand (available via Freightliner Service Pro or aftermarket manuals like Mitchell1), follow this voltage drop test: Avoid generic "free diagram" websites
Freightliner M2 AC Troubleshooting: Fuse, Relay ... - JustAnswer Relay ... - JustAnswer