Hi, don't know about your Sonoma, but on my S10, there's a dropping resistor assembly mounted on top of the evaporator and blower assy, just to the right of and behind the accumulator. (big shiny aluminum cylinder) The resistor pack is mounted right on top of the case, easy to get to. Probably the resistor assembly because it fails on more than one switch position.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C