Place a stethescope or screwdriver or a sensitive ear to the servo u.nit under the hood. With the key on engine off (to facilitate hearing) have someone run the temp from cold to hot. Listen to whether the clockwork mechanism is driven by the internal motor.

If it doesn't then it probably will have a stuck servo but that also can take out the amp. Tell me what you find.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician