I too am trying to make use of this existing wire. Modshack's pic shows the driver's side fuse panel (which a great shot of how to install an add-a-circuit!) but not the harness from the rear fog light. Does the existing fog light harness end in the corresponding spot on the passenger side?
Using an existing (currently useless) wiring harness will save me lots of time and energy in wiring up my rear fog light.
Edit: Weird, but I found the answer to my question under a search for "heated seat switch" (thanks to wstar for his post here
Rear Fog Light Install?). The rear harness apparently hooks up to the BCM behind the kick cover in the p-side footwell, pin #24)!
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Rain, snow, sleet, hail: 2009 Mazda 3 sGT HB
Everything else: 2009 370z, PW Touring 6MT