Does the center air vent drop out when you change from cool to hot setting? One needs to evaluate if the system is moving and that is one good way. If the air is there in the low temp position and goes away in the high temp position, be sure you have the comp switch off. If you still have no heat then there is an adjustment on the temp wheel to get more movement out of the servo but if its gone as far as its going the servo is a common prob.

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