Originally Posted by Brian Carlton

I'd need confirmation for the W123, but the W126 sends 12V to the motor on both lines when the switch is in neutral.

Whether it does this is not all that critical. Could be a different switch design. When you get the 12V with the switch depressed, do you get 12V on a single line or both lines? If you get it on only one line, check the other line for ground. In fact, check for 12V using one line as supply and the other line for ground.

If you get 12V, then make the test again by reversing the switch to the opposite position. The 12V and ground should now switch to the opposite leads.