There have been instances where the wires in the flex-tube that carries current between the door and the door pillar have been work-hardened and become intermittent. Try operating the switches with the door open, to see if that helps - if yes, the wires can be cut and re-spliced where the break is occurring, if not then you could have bad (or dirty) switches.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!