The relay is locate inside your fuse box under hood you may have unscrew the cover to find the relays....auxiliary fan switch is on your drier (color red or green with two wires).....preresistor has two wires connect to each side check it if only side has power
other side not replace it resistance should
be 0 if is open replace it.

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MB STARS Master Guild Technician,12 years MB tech
5 years independent shop owner