Here's how you would configure the wiring for a center-off SPDT dual feed setup:

You'll note that what the SPDT gives you is flexibility.

I personally wanted even more control so I put an illuminated SPST in-line with the feed to the relay. This arrangement gives me an added sense of security (to prevent an inadvertent dead battery) and it also lets me know at a glance that the fogs are on.

Here's what an SPDT/SPST tandem configuration looks like:

The SPDT allows you to select the control tap feed (either low-beam or parking lights), and the SPST turns the fogs on/off while giving you an indicator light while they're on.

Like I said before, this isn't rocket science but hopefully you found it interesting and useful. I know that it's given me a considerable amount of control over my auxiliary lighting package…

Drive safe.