The first thing to do is a little observation. Does the compressor quit? Do the doors audibly change position (audible because if the blower is dead for any reason there will be little air coming out unless it's forced air from driving). if driving there will still be forced air is it still cold, does it heat up?

Definitively, look to the x64 three wire connevtor on the firewalll and verify 1-5v signal (varying with blower speed chosen) on the WT/RD wire. Verify 12v on the power source wire BK/GN and ground on the brown wire. If the power stays on and the ground stays on and there is 1-5v on the control wire the system is doing everything it can to turn on the blower. If it is off then one needs to remove the covers and check a little further to tell if the problem is the blower or the blower controller (all down under the wipers in the engine compartment).

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