1) Confirm that when the switch is in "neutral", that both hot legs at the junction block in the door are at battery voltage.
2) Confirm that the ground leg has zero resistance (terminal-to-battery neg.). Note: The ground leg is tested at the switch.
3) If 1 & 2 are OK, and there is low voltage on one hot leg when the switch is actuated, look to the switch for a high internal resistance, or possibly a damaged wire in the harness where it passes thru the door-to-body area.