You don't need a volt meter, the traces on the PCBoard are so huge,
you can follow them right to the plug. ONE ground feeds every single
light socket. The other two will trace right back to the plug.
I mean, here, look at it:
Disregard the text, but it's so easy to locate the wires for which bulbs. You can see the one
ground... The forth position and socket to it, is not used, it's rear fogs, which is in Euro models.
Where I have the text, and what it's pointing to, is the REVERSE position.