If you lower the panel on the passenger side of the car underneath the glovebox the PES is located on the panel, you should be able to see the unit there - it is a black plastic cased unit, should have either the white or blue label attached.

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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